CI: Only test Cabal lib on GHC 7.4.

.travis.yml

@@ -13,7 +13,7 @@ matrix:
      os: linux
    # These don't have -dyn/-prof whitelisted yet, so we have to
    # do the old-style installation
-   - env: GHCVER=7.4.2 SCRIPT=script
+   - env: GHCVER=7.4.2 SCRIPT=script CABAL_LIB_ONLY=YES
      os: linux
      sudo: required
    - env: GHCVER=7.6.3 SCRIPT=script

travis-script.sh

@@ -73,6 +73,10 @@ timed cabal new-build Cabal Cabal:package-tests Cabal:unit-tests
 
 unset CABAL_BUILDDIR
 
+if [ "x$CABAL_LIB_ONLY" = "xYES" ]; then
+    exit 0;
+fi
+
 # ---------------------------------------------------------------------
 # cabal-install
 # ---------------------------------------------------------------------