Add Google Maven Central repository mirror

[Sylwester Lachiewicz]

Add new mirror as posted by Google.

http://googlecloudplatform.blogspot.com/2015/11/faster-builds-for-Java-developers-with-Maven-Central-mirror.html

For future, this should add faster updates for new package versions.

[Robert Reiz]

Hi @slachiewicz. Many Thanks for this suggestion. Do you use that repo in your maven projects? How current it is? In the comments on the blog post some people write that it's not in sync with Maven central. What is your experience?