Spec: add a failing spec

Some docs are still missing
[GEM] Development dependency on "latest_ruby"
.gitignore: ignore `*~` files
PryDocSpec: the requirement is not really needed
PryDocSpec: move the test to the proper section
PryDocSpec: change the odd test

Pry Doc should be able to find the C stdlib methods. Also, return back
the BigDecimal requirement, since I was wrong. You do have to "require"
stuff in order to get their docs.
Rakefile: automate generation of docs for 1.9 and 2.0

There is a problem with YARD and Ruby 1.8, which I described in
4d01abe7d15154cb29c0a56f750e8da9cc05d245. It could be automated, but
Ruby support will be disbanded soon, so I decided not to bother.