get_timeline.rb

require "twitter"

client = Twitter::REST::Client.new do |config|
  config.consumer_key = "<your consumer key>"
  config.consumer_secret = "<your consumer secret>"
  config.access_token = "<your access token>"
  config.access_token_secret = "<your access token asecret>"
end

client_streaming = Twitter::Streaming::Client.new do |config|
  config.consumer_key = "<your consumer key>"
  config.consumer_secret = "<your consumer secret>"
  config.access_token = "<your access token>"
  config.access_token_secret = "<your access token asecret>"
end

target = client.user("<@Screen Name>") #@で始まる方の名前

def getTweet
  client.user_timeline(target.id, {count: 1}).each do |tweets| #最新1件のツイートを取得
    tweet = client.status(tweets.id, tweet_mode: "extended") #extended -> ツイート全文
    text = tweet.attrs[:full_text] #:full_text -> ツイート全文
    return text
  end
end

client_streaming.user do |object|
  case object
    when Twitter::Tweet
    if(object.user.name == target.name) then
      text = getTweet #object.textの代わりに、上記で定義した関数を置く
      puts text
    end
  end
end