Using Forge's built-in update checker

ClientUpdateHandler.java

package core.upcraftlp.craftdev.api.util;

import com.google.common.collect.Lists;
import core.upcraftlp.craftdev.common.CraftDevCore;
import core.upcraftlp.craftdev.config.CoreInternalConfig;
import net.minecraft.util.StringUtils;
import net.minecraftforge.common.ForgeVersion;
import net.minecraftforge.fml.common.FMLCommonHandler;

import java.util.List;

/**
 * @author UpcraftLP
 * Check for mod updates or register them for automated update announcements
 */
public class UpdateChecker {

    public static final List<String> modsToCheck = Lists.newArrayList();

    /**
     * used to register a mod for update notifications
     * @param modid the modid String
     */
    public static void registerMod(String modid) {
        modsToCheck.add(modid);
    }

    public static boolean hasUpdate(String modid, boolean betaVersions) {
        ForgeVersion.CheckResult result = getResult(modid);
        if(result.status == ForgeVersion.Status.PENDING) {
            CraftDevCore.getLogger().warn("Cannot check for updates, found status: PENDING!");
            return false;
        }
        return betaVersions ? result.status.isAnimated() : result.status == ForgeVersion.Status.OUTDATED;
    }

    public static ForgeVersion.CheckResult getResult(String modid) {
        return ForgeVersion.getResult(FMLCommonHandler.instance().findContainerFor(modid));
    }

    public static String getLatestVersion(String modid) {
        return getResult(modid).target.toString();
    }

    public static void notifyServer() {
        for (String modid : modsToCheck) {
            if (hasUpdate(modid, CoreInternalConfig.betaUpdates)) {
                String url = getResult(modid).url;
                CraftDevCore.getLogger().warn("There's an update available for {}" + (StringUtils.isNullOrEmpty(url) ? "": ", download version {} here: {}"), FMLCommonHandler.instance().findContainerFor(modid).getName(), getLatestVersion(modid), url);
            }
        }
    }
}

UpdateChecker.java

package core.upcraftlp.craftdev.client;

import core.upcraftlp.craftdev.api.util.UpdateChecker;
import core.upcraftlp.craftdev.common.CraftDevReference;
import core.upcraftlp.craftdev.config.CoreInternalConfig;
import net.minecraft.client.Minecraft;
import net.minecraft.util.text.ITextComponent;
import net.minecraft.util.text.Style;
import net.minecraft.util.text.TextComponentString;
import net.minecraft.util.text.TextFormatting;
import net.minecraft.util.text.event.ClickEvent;
import net.minecraft.util.text.event.HoverEvent;
import net.minecraftforge.fml.common.FMLCommonHandler;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
import net.minecraftforge.fml.common.gameevent.TickEvent;
import net.minecraftforge.fml.common.network.FMLNetworkEvent;
import net.minecraftforge.fml.relauncher.Side;
import net.minecraftforge.fml.relauncher.SideOnly;

/**
 * @author UpcraftLP
 */
@SideOnly(Side.CLIENT)
@Mod.EventBusSubscriber(modid = CraftDevReference.MODID, value = {Side.CLIENT})
public class ClientUpdateHandler {

    private static boolean hasShown = false;

    @SubscribeEvent
    public static void onPlayerTick(TickEvent.ClientTickEvent event) {
        if(hasShown || Minecraft.getMinecraft().player == null) return;
        for (String modid : UpdateChecker.modsToCheck) {
            if (UpdateChecker.hasUpdate(modid, CoreInternalConfig.betaUpdates)) {
                String url = UpdateChecker.getResult(modid).url;
                String targetVersion = UpdateChecker.getLatestVersion(modid);
                ITextComponent link = new TextComponentString("here").setStyle(new Style().setColor(TextFormatting.BLUE).setClickEvent(new ClickEvent(ClickEvent.Action.OPEN_URL, url)).setHoverEvent(new HoverEvent(HoverEvent.Action.SHOW_TEXT, new TextComponentString(url).setStyle(new Style().setColor(TextFormatting.AQUA).setItalic(true)))));
                Minecraft.getMinecraft().player.sendMessage(new TextComponentString("Update available for " + FMLCommonHandler.instance().findContainerFor(modid).getName() + ", download v" + targetVersion + " ").appendSibling(link));
            }
        }
        hasShown = true;
    }

    @SubscribeEvent
    public static void onPlayerJoinServer(FMLNetworkEvent.ClientConnectedToServerEvent event) {
        hasShown = false;
    }
}