Fix specs on Ruby 1.9
Add pry as a development dependency
Fix define_method with blocks in Ruby 1.8 and earlier

This is required to support Ruby 1.8 and earlier, which doesn't allow
define_method to take a block that takes a block argument.

See http://coderrr.wordpress.com/2008/10/29/using-define_method-with-blocks-in-ruby-18/
Add Ruby version guards for explicit conversion specifications
Refactor ConversionsModule to be an actual Module

ConversionsModule was previously a class that needed to be initialized
with two arguments: null_class and null_equivs.

Instead, we these arguments are passed via Module#included by setting a
constant on the null class, including the module in the null class, and
then removing the constant and undefining the included methods.
Test against various Ruby implementations