Add Travis configuration file

[tianon]

This is pretty basic, but it's a good start IMO. As-is, it'll test whether we still successfully compile for the three platform+arch combinations we officially support, and we can grow it from there.

The sudo: false bit might not work immediately after enabling Travis for the boot2docker-cli repo (which can be enabled by a repo administrator at https://travis-ci.org/profile/boot2docker), but it might/should work soon (I've requested that the boot2docker org be added to the beta testers list for that).

[tianon]

Ok, Travis is now enabled here, and we've got some Docker-based worker love too (so they start up and run superfast too), so this is officially ready for review. :)

Be sure to check out https://travis-ci.org/boot2docker/boot2docker-cli/builds/29655091 for the "build matrix" details. :D

[SvenDowideit]

very LGTM :)

[tianon]

So, I was digging some more, and I found out that gvm sucks and makes our GOOS and GOARCH build matrix useless in this current form... We're essentially just compiling linux/amd64 three times here. I'm trying to work with @joshk to see if we can find a clean solution for the general case, but otherwise I may just hack this YAML file to work around the issue so we can have our pretties.

[tianon]

Ok, refactored based on what we learned in libcontainer's .travis.yml - ready for proper review.