add OS X travis build (WIP)

Adds support for OS X builds on Travis, closing #414.

TODOs:

- [ ] Download the approprate version of miniconda by checking the [travis OS environment variable] (http://docs.travis-ci.com/user/multi-os/#TRAVIS_OS_NAME-environment-variable).
- [ ] Maybe move to using accelerate (storing our auth token in a [travis encryption key](http://docs.travis-ci.com/user/encryption-keys/). Standardise tests on there.