conditionally build the test suite

GHC 7.10 requires too much memory and time to do this on travis

.travis.yml

@@ -1,18 +1,20 @@
+# GHC 7.10 is too slow to successfully build the test suite
+
 env:
- - GHCVER=7.0.1 CABALVER=1.16
- - GHCVER=7.0.4 CABALVER=1.16
+ - GHCVER=7.0.1 CABALVER=1.16 TESTS="--enable-tests"
+ - GHCVER=7.0.4 CABALVER=1.16 TESTS="--enable-tests"
  # we have to use CABALVER=1.16 for GHC<7.6 as well, as there's
  # no package for earlier cabal versions in the PPA
- - GHCVER=7.2.2 CABALVER=1.16
- - GHCVER=7.4.2 CABALVER=1.16
- - GHCVER=7.6.3 CABALVER=1.16
- - GHCVER=7.8.4 CABALVER=1.18
- - GHCVER=7.10.2 CABALVER=1.22
- - GHCVER=head  CABALVER=1.20
+ - GHCVER=7.2.2 CABALVER=1.16 TESTS="--enable-tests"
+ - GHCVER=7.4.2 CABALVER=1.16 TESTS="--enable-tests"
+ - GHCVER=7.6.3 CABALVER=1.16 TESTS="--enable-tests"
+ - GHCVER=7.8.4 CABALVER=1.18 TESTS="--enable-tests"
+ - GHCVER=7.10.2 CABALVER=1.22 TESTS=""
+ - GHCVER=head CABALVER=head TESTS=""
 
 matrix:
   allow_failures:
-   - env: GHCVER=head  CABALVER=1.20
+   - env: GHCVER=head  CABALVER=head
 
 before_install:
  - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
@@ -23,10 +25,10 @@ before_install:
 
 install:
  - travis_retry cabal update
- - cabal install --enable-tests --only-dependencies
+ - cabal install $TESTS --only-dependencies
 
 script:
- - cabal configure -v2 --enable-tests
+ - cabal configure -v2 $TESTS
  - cabal build
  - cabal sdist
  - export SRC_TGZ=$(cabal info . | awk '{print $2 ".tar.gz";exit}') ;