Use cabal-install 1.20 instead of 1.18 on Travis.

.travis.yml

@@ -11,8 +11,8 @@ env:
 before_install:
  - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
  - travis_retry sudo apt-get update
- - travis_retry sudo apt-get install cabal-install-1.18 ghc-$GHCVER-prof ghc-$GHCVER-dyn happy
- - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/1.18/bin:$PATH
+ - travis_retry sudo apt-get install cabal-install-1.20 ghc-$GHCVER-prof ghc-$GHCVER-dyn happy
+ - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/1.20/bin:$PATH
 
 install:
  - sudo /opt/ghc/$GHCVER/bin/ghc-pkg recache