Add travis-ci continuous builds of firmware

[mithro]

Places to take inspiration from;

• Original HDMI2USB firmware - https://github.com/timvideos/HDMI2USB/wiki/Continuous-Integration
• Atriq - https://github.com/m-labs/artiq/blob/master/.travis.yml

Things to investigate;

• Binstar / Conda
• strace - https://github.com/m-labs/migen/blob/master/tools/strace_tailor.sh
• Joel's fuse system - http://logs.timvideos.us/%23timvideos/%23timvideos.2014-06-03.log.html#t2014-06-03T14:47:55

[mithro]

Started some stuff here -> #43