Try travis with ghc 8 release and with GHCJS (ghc-7.10)

This also exercises 32-bit support, but we really want to do this
explicitly in #9.

.travis.yml

@@ -4,29 +4,38 @@ language: c
 # explicitly request container-based infrastructure
 sudo: false
 
+cache:
+ directories:
+ - $HOME/.cabal
+
 matrix:
   include:
     - env: CABALVER=1.18 GHCVER=7.6.3
       addons: {apt: {packages: [cabal-install-1.18,ghc-7.6.3], sources: [hvr-ghc]}}
     - env: CABALVER=1.18 GHCVER=7.8.4
       addons: {apt: {packages: [cabal-install-1.18,ghc-7.8.4], sources: [hvr-ghc]}}
-    - env: CABALVER=1.22 GHCVER=7.10.1
-      addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.1],sources: [hvr-ghc]}}
+    - env: CABALVER=1.22 GHCVER=7.10.3
+      addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.3],sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.10.3 MAYBE_GHCJS=--ghcjs
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.10.3,happy-1.19.5],sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=8.0.1
+      addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1], sources: [hvr-ghc]}}
     - env: CABALVER=head GHCVER=head
       addons: {apt: {packages: [cabal-install-head,ghc-head],  sources: [hvr-ghc]}}
 
   allow_failures:
    - env: CABALVER=head GHCVER=head
 
 before_install:
- - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+ - export PATH=$HOME/.cabal/bin:/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
+ - test -x $HOME/.cabal/bin/ghcjs  || (cabal install http://ghcjs.luite.com/master.tar.gz && ghcjs-boot)
 
 install:
  - travis_retry cabal update
  - sed -i 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config
- - cabal install --only-dependencies --enable-tests -fdev
+ - cabal install $MAYBE_GHCJS --only-dependencies --enable-tests -fdev
 
 script:
- - cabal configure -fdev --enable-tests -finstrumented
+ - cabal configure $MAYBE_GHCJS -fdev --enable-tests -finstrumented
  - cabal build
  - cabal test tests