Using cabal-install-1.20 on Travis-CI

Though Travis-CI uses cabal-install-1.18, I wanted to use 1.20 to use --allow-newer option. After browsing some documents, the easiest way seems to use ppa:hvr/ghc. You can install cabal-install-1.20 from this ppa, as well as alex and happy.

One thing to note is that you need to install Cabal library explicitly before install packages. Otherwise, it fails to configure packages with not-so-clear errors.

My .travis.yml looks like this.

language: haskell
ghc:
  - 7.8

before_install:
  - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
  - travis_retry sudo apt-get update
  - travis_retry sudo apt-get install --force-yes happy-1.19.4 alex-3.1.3 cabal-install-1.20
  - export PATH=/opt/alex/3.1.3/bin:/opt/happy/1.19.4/bin:/opt/cabal/1.20/bin:$PATH
  - cabal update
  - cabal install Cabal

install:
  - cabal install --only-dependencies --enable-tests --reorder-goals

script:
  - cabal configure --enable-tests && cabal build && cabal test