Remove use of &/to_proc syntactic sugar since it causes RubyMotion to leak

I've submitted the issue with the RubyMotion team and I'm sure they'll look into it and address it. Until then this pull request removes the use of the &/to_proc syntactic sugar as it causes a <code>malloc</code> leak in the underlying RubyMotion code.

It leaks 16 bytes each time sub_render is called. Admittedly, that isn't too much. _But_, it is still a leak.