Add 'about' and 'versions' gradle plugins.

The about plugin [1] includes an about.txt file, containing build environment information, in generated jars.

The versions plugin [2] reports on the freshness of our dependencies.

[1] http://evgeny-goldin.com/wiki/Gradle-about-plugin
[2] https://github.com/ben-manes/gradle-versions-plugin

Reviewed at https://reviews.apache.org/r/16343/