Add travis

I have tested this in my own branch. If you merge this you'll have to flip the switch on travis-ci.org first for this project.

I am also unsure which ruby versions should be supported. Haven't read on that for FFI yet.

Also, should we test against the latest release or against nanomsg master? I would like to make a failing test for nn_poll but the nn_errno is broken in 0.3 and has been fixed in master. ( https://github.com/nanomsg/nanomsg/commit/9f0e4a8be1f4a9f8418ef3252380ab7b9e837155 )