Pin GitX (rowanj fork) to version 0.15.1949

This pull request pins this cask to the most recent stable release of rowanj's fork of GitX.

Previously, this cask used the latest build from master, which is currently unstable on OS X 10.10:
* https://github.com/rowanj/gitx/issues/391
* https://github.com/rowanj/gitx/issues/395
* https://github.com/rowanj/gitx/issues/396
* https://github.com/rowanj/gitx/issues/397
* https://github.com/rowanj/gitx/issues/398