Fix travis build for ghc-7.6.3

Cabal sandboxes were introduced in cabal-install 1.18.

.travis.yml

@@ -8,7 +8,7 @@ dist: trusty
 matrix:
   include:
     - env: CABALVER=1.16 GHCVER=7.6.3
-      addons: {apt: {packages: [cabal-install-1.16,ghc-7.6.3], sources: [hvr-ghc]}}
+      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]}}
     - env: CABALVER=1.24 GHCVER=7.10.2