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.