travis: Add inital support

* Initial support for travis-ci.org
travis: Create build matrix for each target

* Build for each target
* Remove the `-j` flag due to each build VM only having "1.5" cores
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.
Merge pull request #223 from kylemanna/travis

travis: Add inital support for travis-ci.org