Refrect RuboCop changing Rubocop to RuboCop
Fix RuboCop version to 0.24.1 and fix new indications by RuboCop
Fix incorrect description in Markdown
Fix to show failure messages correctly

Before:
 expect to ... but did.
 expect not to ... but did not.
After:
 expect to ... but did not.
 expect not to ... but did.