always wrap component in class-style component
always wrap component in class-style component

Used to support both -- functions would wrap in functions and class
would wrap in classes.  Tests all still pass if we just wrap in class.

There was some issue with typescript compilation, where even class-style
components were being interpreted by styleable as being functions when
they weren't, see: https://github.com/pluralsight/react-styleable/issues/12

This shouldn't be a breaking change for anyone (at least according to
the test suite).  I can't think of other reasons any client would care
about the implementation of the HoC as a class or function.

Closes #12

pluralsight/react-styleable