Refactor Travis and test code

Travis was running into problems like #43, since I reworked the test to run from within one julia process. This restores the previous behaviour on Travis while reducing the overhead on a developer machine to only two julia processes. 

The Travis behaviour is enabled by setting the TRAVIS environment variable to something.

Also added is the infrastructure to test on Julia releases as well as nightlies and to activate OSX testing

The Travis test error is due to julia-release still being at 0.2 on travis