travis: drop 2.2 and include 2.6
travis: drop 2.2 and include 2.6

The 2.2 version isn't working on Travis anymore and it's too old for us to care
about anyway.

Since the 2.6 version has been released, include that one while here.

libgit2/rugged,libgit2/rugged,libgit2/rugged