add support for Rails prompt on v0.12+

[0x1eef]

I would have wished this could have been as simple as:
Pry::Prompt.add 'rails', 'description', PROMPTS

but because of how the public prompt API was implemented, it has
to be like this.

[0x1eef]

@kyrylo @rf-

[rf-]

OK, I just released 0.3.7 with this PR plus some other small fixes.

[falak]

Just do
bundle update pry-rails
will remove the warning.

[pdobb]

The documentation seems to need an update related to this change. It says that, to get the rails prompt do:

if defined?(PryRails::RAILS_PROMPT)
  Pry.config.prompt = PryRails::RAILS_PROMPT
end

But this gem no longer defines PryRails::RAILS_PROMPT. I'm trying to sort out what is the right way to enable the prompt now. Will update if I figure it out, but any help is appreciated.

UPDATE

By reading https://github.com/pry/pry/pull/1849/files I've been able to determine that this should be a suitable replacement for the above in the .pryrc file:

if Pry::Prompt.all.key?("rails")
  Pry.prompt = Pry::Prompt["rails"][:value]
end

It's also nice to know that, within a Pry session, one has access to:

change-prompt --list
change-prompt simple
change-prompt default
change-prompt rails
# etc