Upload coveralls data only when sucess

This is done by moving coveralls send to after_success section
in .travis.yml.
having it in after_success should not make the build fail if
coveralls send fails due to server being overloaded etc.

.travis.yml

@@ -13,6 +13,8 @@ script:
   - make rebar-eunit
   - make rebar-ct
   - make rebar-cover
+
+after_success:
   - make coveralls
 
 sudo: false