Revert "Uncomment mutant in shared Gemspec"

This reverts commit ec1f808d114695b28b9c95c8d12496c8fbfd6a2c.

* The reason for this commit is that mutant needs a new release
  before the dependencies can be satisfied.
Update mutant check in rake task to handle previous installs

* If mutant was previously installed, the binary could still exist.
  Testing it's version should help determine if mutant really is
  available or not.