Use ExecutorService intead of ThreadPoolExecutor in AsyncHttpClient

Changed the setter to use ExecutorService because that's its threadPool
member type and Executors.newFixedThreadPool() returns an ExecutorService
(this method is referenced from the setThreadPoll() javadoc)
Merge pull request #543 from erikghonyan/master

Thanks @erikghonyan, this is indeed better.
Updated build (both Travis and Gradle configuration)
Merge remote-tracking branch 'origin/master'
Merge commit '1bdde80739e5f54d23750cfb89c14138ba5bf505'