Improve the caveat at the top of the readme

[ci skip]
Move the instructions on how to enable native transport

It's a long time since 1.2.5 came out now, for those still running it they can find the instructions under troubleshooting.

[ci skip]
Add instructions about installing snappy

[ci skip]
Set rbx as an allowed failure when running on Travis

Because RVM can't install it for me locally, and it fails on things that shouldn't fail. I don't have time to push up to Travis to test if a workaround works.