Travis.yml now tests multiple versions of GHC.

Suggestions from https://github.com/hvr/multi-ghc-travis

Author: HeinrichApfelmus

.travis.yml

@@ -1,10 +1,29 @@
-language: haskell
+# I do not wish to receive email notifications
 notifications:
   email: false
+
+# NB: don't set `language: haskell` here
+
+# The following enables several GHC versions to be tested; often it's enough to test only against the last release in a major GHC version. Feel free to omit lines listings versions you don't need/want testing for.
+env:
+ - CABALVER=1.18 GHCVER=7.6.3
+ - CABALVER=1.18 GHCVER=7.8.3
+ - CABALVER=1.22 GHCVER=7.10.1
+
+# Note: the distinction between `before_install` and `install` is not important.
 before_install:
-  - cd reactive-banana/
+ - 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 # see note about happy/alex
+ - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+
 install:
-  - cabal install --only-dependencies --enable-tests --force-reinstalls
+ - cabal --version
+ - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
+ - travis_retry cabal update
+ - cd reactive-banana/
+ - cabal install --only-dependencies --enable-tests --enable-benchmarks
+
 before_script:
   - cabal configure
   - cabal sdist