stop using deprecated Class::MOP::load_class

Hello,

Elastic::Model emits deprecation warnings when used with the latest Moose (2.1200), because of it's use of Class::MOP::load_class.  This patch replaces Class::MOP::load_class with Class::Load::load_class. See RT#91002  (https://rt.cpan.org/Public/Bug/Display.html?id=91002).  Please let me know if you'd like me to make any changes and have a great day!

Cheers,
Fitz