Travis: Switch to container-based builds.

Get cabal, alex, and happy from hvr's PPA, and get arm GCC from tarball.

.travis.yml

@@ -1,8 +1,20 @@
+sudo: false
 language: haskell
+addons:
+  apt:
+    sources:
+      - hvr-ghc
+    packages:
+      - cabal-install-1.22
+      - alex-3.1.4
+      - happy-1.19.5
+      - lib32ncurses5
 before_install:
-- sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
-- sudo apt-get update -qq
-- sudo apt-get install -y gcc-arm-none-eabi
+- mkdir -p gcc-arm-embedded
+- wget https://launchpad.net/gcc-arm-embedded/4.9/4.9-2015-q2-update/+download/gcc-arm-none-eabi-4_9-2015q2-20150609-linux.tar.bz2 -O- | tar xjf - -C gcc-arm-embedded
+- export PATH="$(echo $PWD/gcc-arm-embedded/*/bin):/opt/cabal/1.22/bin:/opt/alex/3.1.4/bin:/opt/happy/1.19.5/bin:$PATH"
+- arm-none-eabi-gcc --version && cabal --version && alex --version && happy --version
+install:
 - git clone https://github.com/galoisinc/ivory
 - export IVORY_REPO=$PWD/ivory
 - git clone https://github.com/galoisinc/tower
@@ -11,13 +23,7 @@ before_install:
 - export BSP_REPO=$PWD/ivory-tower-stm32
 - git clone https://github.com/aisamanra/s-cargot
 - export SCARGOT_REPO=$PWD/s-cargot
-install:
-- cabal install cabal-install
-- cabal install alex happy
 script:
-- ghc --version
-- cabal --version
-- arm-none-eabi-gcc --version
 - make create-sandbox
 - make
 - make $ENV_TEST_TARGET