Install happy for style builds.

Motivated from this build failure on HEAD:

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

Related to #3178.

This fix is based on a comments from:

  * #3178 (comment)
  * #3151 (comment)

.travis.yml

@@ -78,6 +78,10 @@ install:
    set -ex
    case "$BUILD" in
      style)
+       # This should be resolved (and removed) by Stack 1.6
+       # Follows from https://github.com/commercialhaskell/stack/issues/3178
+       stack --system-ghc --no-terminal install happy
+
        stack --system-ghc --no-terminal install hlint
        ;;
      stack)