Updated shape to use style method

Hi again all!

This one involved a little more tweaking than the other ones. It looks like the configuration specs were using Shape (randomly?) so I had to update them to not look for a style hash since shape doesn't use it.

Tests pass and everything else was straight forward so I think this is ready to go. 

PS. I'll merge this (and others) after at least one reviewer gives me the go ahead as per (1) [here](https://github.com/shoes/shoes4/wiki/Merging-a-Pull-Request).
Good catch! The better solution would be to make Shoes::Gradient behave as expected when we test for equality. This means overriding the spaceship operator `<=>`. See Shoes::Color for an example.