fix happy version

Author: tittoassini

.travis.yml

@@ -23,7 +23,7 @@ matrix:
 
   - env: BUILD=cabal GHCVER=8.4.2 CABALVER=2.2
     compiler: ": #GHC 8.4.2"
-    addons: {apt: {packages: [cabal-install-2.2,happy-1.19.9,ghc-8.4.2], sources: [hvr-ghc]}}
+    addons: {apt: {packages: [cabal-install-2.2,happy-1.19.5,ghc-8.4.2], sources: [hvr-ghc]}}
 
   # - env: BUILD=stack ARGS="--resolver lts-2"
   #   compiler: ": #stack 7.8.4"