http://blog.synchack.com/2012/02/blog-post_18.html

getVersionName.java

String getVersionName(Context context) {
    String versionStr = "(none)";
    PackageManager pm = context.getPackageManager();
    try {
        PackageInfo info = pm.getPackageInfo(context.getPackageName(), 0);
        versionStr = info.versionName;
    } catch (Exception e) {
        // no mentenance...
    }
    return versionStr;
}