Drop rainbow

This takes advantage of previous pull request (Please pull in #89 first).  (to put in semantic messages over hard coded colour).

It makes the simple formatter just render ascii colour codes itself, and thus drops the dependency of rainbow, removing the need to sort #83 given what we actually need.

I think it might not respect terminal hints though, so could mean Jenkins sees raw codes, not sure if it's worth adding in though.  This should also solve #77