Set GIT_AUTHOR_NAME in tests using git.

Travis is useless if every build fails. This pull request fixes that.

Because of the way that Travis CI is set up, the Git user isn't
configured for committing (user.name, user.email, Full Name in
/etc/passwd). Explicitly setting GIT_AUTHOR_NAME ensures that the tests
involving Git don't fail due to problems with Travis.

To test locally, try running

    GIT_AUTHOR_NAME="" bundle exec rspec -bfs spec

with this branch compared to earlier commits.

To see more information about the error, see the [Git FAQ]
(https://git.wiki.kernel.org/index.php/Git_FAQ#Git_commit_is_dying_telling_me_.22fatal:_empty_ident_.3Cuser.40myhost.3E_not_allowed.22.2C_what.27s_wrong.3F).