Robustify the plugin require

I was getting trouble with both pry-editline and pry-syntax-hacks, while
running with a newly-compiled-Ruby with GNU Readline on a Mac (the
default Readline was working OK with these. There could be more
investigation about the error, but for now, at least pry boots with a
handy error instead of a total failure).
Add a Procfile for testing foreman

The foreman run isn't terribly special, but it's another stdio situation
that people get into in real life.

So this is just a way to check that same scenario. For now, no
automation is in place (or necessary?), but it's something to tinker
with via:

    gem install foreman && cd spec && foreman start
Add Pry::Terminal.height! (only used once)
Reword foreman instructions without 'cd'