travis: only run unit tests

Travis has chronic problems successfully running the integration suite -
and we've successfully moved to Semaphore for that purpose - but can
still be useful as a fail-fast option for testing unit tests and formatting.

.travis.yml

@@ -5,4 +5,4 @@ go:
   - 1.5
 
 script:
- - INTEGRATION=y ./test
+ - ./test