more strict regexpes for highlighting prompts

This is a fix for #601.

The tests I broke actually served a purpose as they set by `rvm`(which is ruby-version manager). I therefore kept the tests and adapted my reg exp.

The reg exp I built is fairly strict consisting of three different reg expes to match each prompt. I tried finding a more general one that covered all three cases while not breaking other highlighting but I couldn't find one.
Drop the `new RegExp` constructor, we accept both literal regexes and strings, and it's going to be reconstructed during language compilation anyway.