Travis: Allow failures on GHC head

.travis.yml

@@ -1,35 +1,39 @@
 sudo: required
 
 env:
- - CABALVER=1.18 GHCVER=7.6.3
- - CABALVER=1.18 GHCVER=7.8.4
- - CABALVER=1.22 GHCVER=7.10.2
- - CABALVER=head GHCVER=head
+  - CABALVER=1.18 GHCVER=7.6.3
+  - CABALVER=1.18 GHCVER=7.8.4
+  - CABALVER=1.22 GHCVER=7.10.2
+  - CABALVER=head GHCVER=head
+
+matrix:
+  allow_failures:
+   - env: CABALVER=head GHCVER=head
 
 # Note: the distinction between `before_install` and `install` is not important.
 before_install:
- - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
- - travis_retry sudo apt-get update
- - travis_retry sudo apt-get install cabal-install-$CABALVER ghc-$GHCVER # see note about happy/alex
- - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+  - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
+  - travis_retry sudo apt-get update
+  - travis_retry sudo apt-get install cabal-install-$CABALVER ghc-$GHCVER # see note about happy/alex
+  - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
 
 install:
- - cabal --version
- - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
- - travis_retry cabal update
- - cabal install --only-dependencies --enable-tests --enable-benchmarks
+  - cabal --version
+  - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
+  - travis_retry cabal update
+  - cabal install --only-dependencies --enable-tests --enable-benchmarks
 
 # Here starts the actual work to be performed for the package under test; any command which exits with a non-zero exit code causes the build to fail.
 script:
- - if [ -f configure.ac ]; then autoreconf -i; fi
- - cabal configure --enable-tests --enable-benchmarks -v2  # -v2 provides useful information for debugging
- - cabal build   # this builds all libraries and executables (including tests/benchmarks)
- - cabal test
- - cabal check
- - cabal sdist   # tests that a source-distribution can be generated
+  - if [ -f configure.ac ]; then autoreconf -i; fi
+  - cabal configure --enable-tests --enable-benchmarks -v2  # -v2 provides useful information for debugging
+  - cabal build   # this builds all libraries and executables (including tests/benchmarks)
+  - cabal test
+  - cabal check
+  - cabal sdist   # tests that a source-distribution can be generated
 
 # Check that the resulting source distribution can be built & installed.
 # If there are no other `.tar.gz` files in `dist`, this can be even simpler:
 # `cabal install --force-reinstalls dist/*-*.tar.gz`
- - SRC_TGZ=$(cabal info . | awk '{print $2;exit}').tar.gz &&
-   (cd dist && cabal install --force-reinstalls "$SRC_TGZ")
+  - SRC_TGZ=$(cabal info . | awk '{print $2;exit}').tar.gz &&
+    (cd dist && cabal install --force-reinstalls "$SRC_TGZ")