CI on pull requests

[Josh Elser]

For those interested in using Github pull requests for code reviews, 
with the help of Chris T from infra, I got a "check" working which 
spawns a job on the builds.a.o Jenkins server.

https://builds.apache.org/job/PreCommit-RATIS-github-pull-requests/
It doesn't look perfect (some test failures on a dummy commit), but it's 
better than nothing ;)