Rewrite .travis.yml to use containers and stack.

This should make the build faster and more reliable,
and hopefully ressurect the test suite.

.travis.yml

-# Taken from multi-ghc-travis
-# NB: don't set `language: haskell` here
-
-# The following enables several GHC versions to be tested; often it's enough to test only against the last release in a major GHC version. Feel free to omit lines listings versions you don't need/want testing for.
+# Test against different stack LTS versions.
 env:
- - CABALVER=1.18 GHCVER=7.6.3
- - CABALVER=1.18 GHCVER=7.8.4 # see note about Alex/Happy for GHC >= 7.8
- - CABALVER=1.22 GHCVER=7.10.1
- - CABALVER=1.22 GHCVER=7.10.2
-# - CABALVER=head GHCVER=head   # see section about GHC HEAD snapshots
+ - LTS=6.2  # Most recent GHC 7.10 LTS
+ - LTS=2.22 # Last GHC 7.8 LTS
+
+# Enable caching.
+sudo: false
+
+# Caching so the next build will be fast too.
+cache:
+  directories:
+  - $HOME/.ghc
+  - $HOME/.cabal
+  - $HOME/.stack
 
-# 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
- - travis_retry sudo apt-get install libmagic-dev
- - travis_retry sudo apt-get install python3
- - travis_retry git clone http://www.github.com/zeromq/zeromq4-x.git libzmq
- - export OLDPWD=$(pwd) && cd libzmq && travis_retry ./autogen.sh && travis_retry ./configure && make && travis_retry sudo make install && travis_retry sudo ldconfig && cd $OLDPWD
- - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$HOME/.cabal/bin:$PATH
- - travis_retry sudo apt-get install happy-1.19.4 alex-3.1.3
- - export PATH=/opt/alex/3.1.3/bin:/opt/happy/1.19.4/bin:$PATH
- - travis_retry sudo apt-get install libblas-dev liblapack-dev
+    # Download and unpack the stack executable
+  - export PATH=$HOME/.local/bin:$PATH
+  - mkdir -p ~/.local/bin
+  - curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'
+
+addons:
+  apt:
+    packages:
+      - libmagic-dev
+      - zeromq3
 
 install:
- - cabal --version
- - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
- - travis_retry cabal update
- - travis_retry cabal install cpphs
- - travis_retry cabal install gtk2hs-buildtools
- - |
-   if [ ${GHCVER%.*} = "7.8" ]; then
-     travis_retry cabal install arithmoi==0.4.* -fllvm
-     travis_retry git clone http://www.github.com/gibiansky/hindent
-     cd hindent && travis_retry cabal install && cd ..
-   fi
+  - stack build
 
-# 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 [[ ${GHCVER%.*} == "7.10" ]]; then
-     travis_retry ./build.sh all
-   else
-     travis_retry ./build.sh ihaskell
-   fi
-
-# Build and run the test suite
- - travis_retry cabal install --only-dependencies --enable-tests
- - travis_retry cabal configure --enable-tests
- - |
-   if [ ${GHCVER%.*} = "7.8" ]; then
-    travis_retry cabal test --show-details=always
-   fi
- - |
-   if [ ${GHCVER%.*} = "7.8" ]; then
-    ./verify_formatting.py
-   fi
- - cabal sdist
-
-# The following scriptlet checks that the resulting source distribution can be built & installed
- - export SRC_TGZ=$(cabal info . | awk '{print $2 ".tar.gz";exit}') ;
-   cd dist/;
-   if [ -f "$SRC_TGZ" ]; then
-      cabal install --force-reinstalls "$SRC_TGZ";
-   else
-      echo "expected '$SRC_TGZ' not found";
-      exit 1;
-   fi
+  - stack test