Try to use CABALVER=1.20 again

...hoping that haskell/cabal#1806 has been resolved for good

Author: hvr

.travis.yml

@@ -8,13 +8,11 @@ env:
  - GHCVER=7.4.2 CABALVER=1.16
  - GHCVER=7.6.3 CABALVER=1.16
  - GHCVER=7.8.2 CABALVER=1.18
- # NOTE: we can't use Cabal 1.20 yet due to
- #  https://github.com/haskell/cabal/issues/1806
- - GHCVER=head  CABALVER=1.18
+ - GHCVER=head  CABALVER=1.20
 
 matrix:
   allow_failures:
-   - env: GHCVER=head  CABALVER=1.18
+   - env: GHCVER=head  CABALVER=1.20
 
 # Note: the distinction between `before_install` and `install` is not
 #       important.