Clean up packaged gem by removing unneeded files.

Fixes #2
Relax development dependencies.

- Allow specs to pass if coveralls is removed manually.
- Remove hard dependency on yard and redcarpet.  They must be installed
  manually to properly generate documentation, but not all developers
  are doing that.

Fixes #3
Merge pull request #9 from ManageIQ/clean_up_gemspec

Clean up gemspec