Handle block param for custom matcher with block_param method

PR for issue #321 
For version-specific stuff, we prefer to evaluate the conditional once at load time rather than each time the method is called.  Do you mind doing this instead?

``` ruby
if Proc.method_defined?(:parameters)
  def warn_about_block_parameters(name, declarations)
    # (the logic you already have minus the `RUBY_VERSION` check)
  end
else # for 1.8
  def warn_about_block_parameters(*)
    # There's no way to detect block params on 1.8 since the method reflection APIs don't expose it
  end
end
```