Centralize cabal-install version.

.travis.yml

@@ -1,10 +1,13 @@
 env:
- - GHCVER=7.0.4
- - GHCVER=7.2.2
- - GHCVER=7.4.2
- - GHCVER=7.6.3
- - GHCVER=7.8.3
- - GHCVER=head
+ global:
+  - CABALVER=7.0.4
+ matrix:
+  - GHCVER=7.0.4
+  - GHCVER=7.2.2
+  - GHCVER=7.4.2
+  - GHCVER=7.6.3
+  - GHCVER=7.8.3
+  - GHCVER=head
 
 matrix:
  allow_failures:
@@ -13,15 +16,15 @@ matrix:
 before_install:
  - sudo add-apt-repository -y ppa:hvr/ghc
  - sudo apt-get update -qq
- - sudo apt-get install -qq cabal-install-1.20 ghc-$GHCVER
+ - sudo apt-get install -qq cabal-install-$CABALVER ghc-$GHCVER
  - export PATH="/opt/ghc/$GHCVER/bin:$PATH"
 
 install:
- - cabal-1.20 update
+ - cabal-$CABALVER update
  - ghc --version
  - ghc-pkg list
 
 script:
- - cabal-1.20 install --enable-documentation
- - cabal-1.20 sdist
+ - cabal-$CABALVER install --enable-documentation
+ - cabal-$CABALVER sdist
  - ghc-pkg list