Remove support for 1.8-compatible versions of Ruby
Fix failing jruby spec
Fix default inspect for Ruby 2.1

In Ruby 2.1, the ancestor list of a singleton class includes the
singleton as the first element, so we need to filter it out.
Disable spec to work around rubinius/rubinius#2871
Merge pull request #1093 from pry/remove_mri18_support

Clean up support for Ruby 1.8