Vagrantfile: Set git config for integration tests

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #573 from flynn/vagrant-git-user

Vagrantfile: Set git config for integration tests
Merge branch 'master' of https://github.com/flynn/flynn
slugbuilder: Add proxy support to custom buildpack downloads

Signed-off-by: Travell Perkins <travell.perkins@fmr.com>