Set GIT_{AUTHOR,COMITTER}_{EMAIL,NAME} for Travis.

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. Explicitly setting
GIT_AUTHOR and GIT_COMMITTER variables 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).