Update to 1.6.1

changes:
1.6.1
  - version 1.6 --> 1.6.1
1.6
  - Disable |Generic1 f => GCopoint f| instance
  - Add generic Copoint
  - Merge pull request https://github.com/dreixel/generic-deriving/issues/9
      Minor modifications for building with latest Template-Haskell 2.9