travis: Use arm-none-eabi-gcc compiler

* Use the gcc-arm-none-eabi compiler.  This will cause Travis-CI to
  set the env variable `CC=gcc-arm-none-eabi` which would allow the
  Makefile to be simplified by not hardcoding `CC`
* "Travis CI VMs run on 1.5 virtual cores"
   http://docs.travis-ci.com/user/speeding-up-the-build/
   Use `-j2`
* Ask GCC to print it's version.  Travis-CI would do this automatically
  if there was a way to install gcc before `$CC --version` is
  automatically run.

.travis.yml

@@ -11,7 +11,8 @@ env:
   - TARGET=SPARKY
   - TARGET=STM32F3DISCOVERY
 language: c
-compiler: gcc
+compiler: arm-none-eabi-gcc
 before_install: sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded && sudo apt-get update
 install: sudo apt-get install build-essential gcc-arm-none-eabi git
-script: make
+before_script: $CC --version
+script: make -j2