Build Particle firmware on travis

[meisteg]

Should be able to build the firmware on travis. The local build steps can be found at:
https://docs.particle.io/faq/particle-tools/local-build/core/#installation-linux

Travis lint tool:
http://lint.travis-ci.org/

The problem is travis doesn't seem to support multiple languages in a single file. The solution see here doesn't pass the lint test:
https://stackoverflow.com/a/44054333/1620158

Something like this should work:

language: cpp
dist: trusty
sudo: false

install:
  - wget https://launchpad.net/gcc-arm-embedded/4.9/4.9-2015-q3-update/+download/gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
  - bzip2 -d gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
  - tar xf gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar
  - export PATH=$PATH:$HOME/gcc-arm-none-eabi-4_9-2015q3/bin
  - arm-none-eabi-gcc --version

before_script:
  - git clone https://github.com/spark/firmware.git -b release/stable

script:
  - cd firmware/main
  - make all PLATFORM=photon APPDIR=$HOME/RaspberryPiTempAlarm/hw/particle