Install ghc-8.2.1 explictily

Author: nomeata

.travis.yml

@@ -18,7 +18,7 @@ addons:
       key_url: 'https://deb.nodesource.com/gpgkey/nodesource.gpg.key'
     packages:
     - cabal-install-2.2
-    - ghc-8.2
+    - ghc-8.2.1
     - ghcjs-8.2
     - alex-3.1.4
     - happy-1.19.5