Fix travis build

Author: hongchangwu

.travis.yml

@@ -1,4 +1,26 @@
-language: haskell
+language: generic
+
+sudo: false
+
+matrix:
+  include:
+    - env: CABALVER=1.18 GHCVER=7.8.4
+      addons: {apt: {packages: [cabal-install-1.18,ghc-7.8.4], sources: [hvr-ghc]}}
+    - env: CABALVER=1.22 GHCVER=7.10.2
+      addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.2],sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=8.0.1
+      addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1], sources: [hvr-ghc]}}
+
+before_install:
+  - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+
+install:
+  - cabal --version
+  - ghc --version
+  - travis_retry cabal update
+  - cabal install --only-dependencies --enable-tests
 
 script:
-  - cabal configure --enable-tests --ghc-options=-Werror && cabal build && cabal test
+  - cabal configure --enable-tests --ghc-options=-Werror
+  - cabal build
+  - cabal test