Install happy for style builds.
Install happy for style builds.

Motivated from this build failure on HEAD:

  * https://travis-ci.org/commercialhaskell/stack/jobs/266365181

Related to https://github.com/commercialhaskell/stack/issues/3178.

This fix is based on a comments from:

  * https://github.com/commercialhaskell/stack/issues/3178#issuecomment-306405297
  * https://github.com/commercialhaskell/stack/issues/3151#issuecomment-313126596

MichielDerhaeg/stack,anton-dessiatov/stack,MichielDerhaeg/stack,juhp/stack,juhp/stack,anton-dessiatov/stack