Add extra support for determining current ruby type

Add `rbx?` `mri?` and `non_mri?` Ruby "types", for rspec/rspec-core#1760 but that highlighted to me that we could use some refactoring across our repos to remove duplication of detection for ruby types
Seems like this could use a feature detection solution as well?  You can use a code snippet like:

``` ruby
begin
  Module.new { def foo; end }.instance_method(:foo).bind(Object.new)
  def supports_rebinding_module_methods?; true; end
rescue TypeError
  def supports_rebinding_module_methods?; false; end
end
```