Enable release and nightlies on travis

This change enables the running of travis tests with both nightly and release versions of Julia. 

Current tests run only using Julia nightly. This PR itself will prove if current StatsBase actually works in 0.2. So lets wait for Travis. 

Note that enabling this kind of testing should not be considered to be a irrevocable promise of supporting release julia by this package. Package maintainers can easily remove tests for older julia. However, these tests will make that decision explicit, and motivate correct dependency declaration via REQUIRE