.travis.yml: Unbreak the build.

[ci skip]

.travis.yml

@@ -57,6 +57,7 @@ script:
 # Also build cabal-install.
  - cd ../cabal-install
  - cabal sandbox init
+ - cabal sandbox add-source ../Cabal
  - cabal install --dependencies-only --enable-tests
  - cabal configure --enable-tests --ghc-option=-Werror
  - cabal build