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
Fix undefined method `[]' for nil:NilClass from rspec -e.

$options was only set in CLI#run so running only non-CLI spec examples
could fail because $options was nil when the array index operator was
called on it.
Merge pull request #107 from jonas054/fix_options_nil

Fix undefined method `[]' for nil:NilClass from rspec -e.
New cop ClassMethods
Show error message alongside backtrace