Add CI verification step about i3status-rust

[greshake]

I'll set it up when I have the time. Currently I check every PR by hand :)

[greshake]

Okay, I've got Travis running now. Do you know how to trigger builds for exisiting PR's right now? I'll just try to trigger it with a dummy commit to one of your PR's.

[pitkley]

Thanks for adding Travis. I see that you have removed beta from the Rust version matrix -- is there a specific reason why you did this? While compatibility with nightly probably includes beta, I would assume that a build failure on beta should not be allowed.

(Not sure about triggering Travis, but you found a workaround and for future PRs it hopefully triggers automatically! 👍)

[greshake]

Yea Travis just didn't trigger because these PR's were created before I set it up. Happens automatically in the future. I removed beta because I assume that failure to build in beta either indicates an error in the beta toolchain (in which case I would have to spend unnessesary resources finding that error) or behaviour which will be deprecated in the future, in which case nightly build failure is a better indicator. Also it might delay builds slightly if multiple branches need to be built. Just didn't see that to be particularly useful.

[greshake]

Completed in 6953f81