Updating git-tf to version 2.0.3.20131219

This git-tf release deprecates support for legacy Microsoft Team Foundation Server versions (anything older to TFS 2010). This caveat information has been added to the formula.

Also, make sure to merge the following in homebrew-versions, which keeps the legacy client around, with support for older versions of TFS:

https://github.com/Homebrew/homebrew-versions/pull/345