Drop Ruby 1.8.7 support
spec/pry-doc_spec.rb: require 'readline' and fix a failing test
Rescan docs for Ruby 2.1
Rescanned docs for Ruby 2.0 (based on p484)
pry-doc.gempsec: write a better description
pry-doc.gemspec: rewrite dependencies versions

RubyGems should stop bitching now.
lib/pry-doc.rb: use different strategy for determining methods

Fixes #20 (Fix 2.1.1 support)

The old strategy doesn't work, because the Ruby authors decided to
change some of its internals. Learn more:

https://bugs.ruby-lang.org/issues/8035

This approach tries to cover all the Rubies. However, I hope to see
`#reciever_notation_for` refactored in 2 years or so :)