set up travis for mac + linux

See http://docs.travis-ci.com/user/multi-os/ - you need to send an email to support@travis-ci.com asking them to enable multi-OS support for this repo. They were short on capacity for a while, but looks like they're accepting new repos again now.

The `language: objective-c` line is temporary just to see what a test build looks like on mac only, we'll change that back for the real thing.