add appraisal gem to test against supported versions of rubocop
update rubocop requirement to 0.14.0 or above
Rename module name from Rubocop to RuboCop to follow RuboCop gem update.

Rubocop module name has changed from following commit.

Rename Rubocop module to RuboCop
  * https://github.com/bbatsov/rubocop/commit/ff167d8f202baf7a68955db0aaf0dc29afb7e7ee