Fix travis typo

.travis.yml

@@ -7,7 +7,7 @@ dist: trusty
 
 matrix:
   include:
-    - env: CABALVER=1.16 GHCVER=7.6.3
+    - env: CABALVER=1.18 GHCVER=7.6.3
       addons: {apt: {packages: [cabal-install-1.18,ghc-7.6.3], sources: [hvr-ghc]}}
     - env: CABALVER=1.22 GHCVER=7.8.4
       addons: {apt: {packages: [cabal-install-1.22,ghc-7.8.4], sources: [hvr-ghc]}}