Run Git-LFS tests against the Travis-CI default Git version and the
latest available Git version on the respective platform.
The default Git version is currently 1.8.5.6 on Ubuntu and 1.9.3 on OSX.
With the latest Git installed (2.7 right now) the following test are executed
in addition:
test/test-credentials-no-prompt.sh (git version < 2.3.0)
test/test-prune-worktree.sh (git version < 2.5.0)
test/test-push-bad-dns.sh (git version < 2.3.0)
test/test-worktree.sh (git version < 2.5.0)
Doesn't usually build with a fork because the clone path is $fork_user/git-lfs instead of github/git-lfs which breaks package references. This fixes that.
Note that this code should work on any golang repo owned by github
