Turn on Travis fast_finish, per https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

.travis.yml

@@ -36,3 +36,4 @@ script:
 matrix:
   allow_failures:
    - env: GHCVER=head
+  fast_finish: true