This patch add support for running against multiple versions of emacs, it currently runs against V23 and V24, example[0]
As a bonus, it looks like the tests are failing using emacs V24[1] [0]: https://travis-ci.org/eMxyzptlk/notmuch/builds/26612550 [1]: https://travis-ci.org/eMxyzptlk/notmuch/jobs/26612552#L882
