Update Buck

This version makes test coverage tool JaCoCo work in the wild [1].

[1] https://github.com/facebook/buck/commit/ab0133d57643925564ee208845dc8568b26b6a2c

Change-Id: I497bd5e5a88bdda1ea6b3766d199d62cb73318ed
Merge "Update Buck"
Fix unparsable JSON in Documentation

Change-Id: Ifab4041d973f2fbc096a25f2bbc96e98a2820a94
Merge "Fix unparsable JSON in Documentation"
Buck: Document how to create test coverage report

Upstream Buck added support for JaCoCo report generation recently.

Document how to invoke test code coverage so that Gerrit devs can
enjoy the reports [1].

[1] http://imgur.com/wSMxIZa

Change-Id: If80bca30d7ba036c15ab3504855d4b0a6f6abc3c