Travis config: fix allow failure with GHC HEAD

Broken in commit e22b619.

Author: thomie

.travis.yml

@@ -14,7 +14,7 @@ env:
 
 matrix:
   allow_failures:
-   - env: GHCVER=head  CABALVER=head
+   - env: GHCVER=head CABALVER=head TESTS=""
 
 before_install:
  - travis_retry sudo add-apt-repository -y ppa:hvr/ghc