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
Merge pull request #4 from tune/rename_module_name

Rename module name from Rubocop to RuboCop to follow RuboCop gem update.