Tweak the .travis.yml file

We want to run 'make tests' and not that rebar command.
Also remove R14B as quoted doesn't work with it.

.travis.yml

@@ -3,4 +3,4 @@ otp_release:
   - R14B03
   - R14B02
   - R14B01
-  - R14B
+script: "make tests"