Update Travis build instructions

Follow "S3 Hackage mirror for Travis
builds" (http://www.yesodweb.com/blog/2015/07/s3-hackage-mirror-travis)
suggestions.

.travis-setup.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+set -eux
+
+mkdir -p $HOME/.cabal
+cat > $HOME/.cabal/config <<EOF
+remote-repo: hackage.haskell.org:http://hackage.fpcomplete.com/
+remote-repo-cache: $HOME/.cabal/packages
+jobs: \$ncpus
+EOF

.travis.yml

@@ -13,6 +13,9 @@ before_install:
  - 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:$HOME/.cabal/bin:$PATH
+ - export PATH=$HOME/.cabal/bin:/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+ - ./.travis-setup.sh
+
 
 install:
  - cabal --version