Update Travis CI Job

.travis.yml

 env:
- - GHCVER=7.4.1
- - GHCVER=7.4.2
- - GHCVER=7.6.1
- - GHCVER=7.6.2
- - GHCVER=7.6.3
+ - CABALVER=1.16 GHCVER=7.4.1
+ - CABALVER=1.16 GHCVER=7.4.2
+ - CABALVER=1.16 GHCVER=7.6.1
+ - CABALVER=1.16 GHCVER=7.6.2
+ - CABALVER=1.16 GHCVER=7.6.3
+ - CABALVER=1.18 GHCVER=7.8.1
+ - CABALVER=1.18 GHCVER=7.8.2
+ - CABALVER=1.18 GHCVER=7.8.3
+ - CABALVER=head GHCVER=head
+
+matrix:
+  allow_failures:
+   - env: CABALVER=head GHCVER=head
 
 before_install:
- - sudo add-apt-repository -y ppa:hvr/ghc
- - sudo apt-get update
- - sudo apt-get install cabal-install-1.18 ghc-$GHCVER autoconf
- - export PATH=/opt/ghc/$GHCVER/bin:$PATH
+ - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
+ - travis_retry sudo apt-get update
+ - travis_retry sudo apt-get install cabal-install-$CABALVER ghc-$GHCVER autoconf
+ - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
 
 install:
- - cabal-1.18 update
- - ghc --version
+ - cabal --version
+ - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
+ - travis_retry cabal update
 
 script:
  - autoreconf -i
- - cabal-1.18 configure -v2
- - cabal-1.18 build
- - cabal-1.18 check
- - cabal-1.18 sdist
- - export SRC_TGZ=$(cabal-1.18 info . | awk '{print $2 ".tar.gz";exit}') ;
+ - cabal configure -v2
+ - cabal build
+ - cabal check
+ - cabal sdist
+ - export SRC_TGZ=$(cabal info . | awk '{print $2 ".tar.gz";exit}') ;
    cd dist/;
    if [ -f "$SRC_TGZ" ]; then
-      cabal-1.18 install "$SRC_TGZ";
+      cabal install --force-reinstalls "$SRC_TGZ";
    else
       echo "expected '$SRC_TGZ' not found";
       exit 1;