[xctool] Pull latest changes to xctool for xcode 5.1 compatibility

Summary:
xctool was broken in xcode 5.1. The most recent changes merged today
fix the build so it'll work for unit tests just like it did before.

Test Plan:
Run `arc unit` in a project that uses cassowary, and see the
build succeeds and unit tests actually run.

Reviewers: edmond, bobo

Reviewed By: bobo

Differential Revision: https://phabricator.robinhood.com/D588