Run tests with older versions of Git

[abless]

Tests currently run with whatever git version is running on Circle (https://circleci.com/docs/1.0/build-image-precise/). We'd want to make sure that tests also pass for older Git versions (at least >=1.7.1 to support CentOS6 images).

[iamdanfox]

@abless I've published https://hub.docker.com/r/iamdanfox/oldgit/ which has git 1.7.1 built from source - we should be able to use this to run a few tests :)