Fix `rake coverage` does not generate coverage report files

This problem is caused by Coveralls.
Coveralls uses SimpleCov internally,
and it changes SimpleCov.formatter to Coverall's formatter
by overwriting standard HTML formatter.
So disable Coveralls except in Travis environment.
Anyway Coveralls does nothing outside of Travis currently.
Exclude spec files from coverage target for accurate coverage result
Merge pull request #106 from yujinakayama/fix-coverage

Fix coverage