--force-yes in Travis script (fixes #3292)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

.travis.yml

@@ -16,8 +16,8 @@ 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-1.24 ghc-$GHCVER-prof ghc-$GHCVER-dyn happy
- - if [ "$TEST_OLDER" == "YES" ]; then travis_retry sudo apt-get install ghc-7.0.4-prof ghc-7.0.4-dyn ghc-7.2.2-prof ghc-7.2.2-dyn; fi
+ - travis_retry sudo apt-get install --force-yes cabal-install-1.24 ghc-$GHCVER-prof ghc-$GHCVER-dyn happy
+ - if [ "$TEST_OLDER" == "YES" ]; then travis_retry sudo apt-get install --force-yes ghc-7.0.4-prof ghc-7.0.4-dyn ghc-7.2.2-prof ghc-7.2.2-dyn; fi
  - export PATH=$HOME/.cabal/bin:/opt/ghc/$GHCVER/bin:/opt/cabal/1.24/bin:$PATH
  - git version