Travis: use ghc-7.8.4 as stage0 to fix the build

.travis.yml

@@ -16,13 +16,13 @@ addons:
     - ubuntu-toolchain-r-test
     packages:
     - cabal-install-1.18
-    - ghc-7.6.3
+    - ghc-7.8.4
     - alex-3.1.3
     - happy-1.19.4
     - llvm-3.6
 
 before_install:
- - export PATH=/opt/ghc/7.6.3/bin:/opt/cabal/1.18/bin:/opt/alex/3.1.3/bin:/opt/happy/1.19.4/bin:/usr/lib/llvm-3.6/bin:$PATH
+ - export PATH=/opt/ghc/7.8.4/bin:/opt/cabal/1.18/bin:/opt/alex/3.1.3/bin:/opt/happy/1.19.4/bin:/usr/lib/llvm-3.6/bin:$PATH
 
 # Be explicit about which protocol to use, such that we don't have to repeat the rewrite command for each.
  - git config remote.origin.url git://github.com/${TRAVIS_REPO_SLUG}.git