Add Travis CI to continually build PRs and master on each push

[ccjoel]

Requires admin permissions on this repo (to activate on travis website). Free for open source projects.

As an example of how useful this would be, I pushed a change to build the jar file in a different way, but broke lein repl by using _ instead of - in the main class name. Travis would have caught this. Instead, it went like that for 3 days until I noticed that I coudn't start a repl.

The travis job would run lein iberjar (maybe on multiple java environments), lein test and lein repl (if we can).