Compile Teensy code on Travis CI

[wchill]

We should test compilation of the Teensy code on Travis CI before accepting PRs to that code.

This is a good starting point:
https://learn.adafruit.com/continuous-integration-arduino-and-you/testing-your-project

Note that the tutorial only installs libraries to support Arduino boards; we have a dependency on Teensyduino to compile Arduino sketches for Teensy LC so this should be taken into consideration.