Fix Holzapfel-Ogden (2009) constitutive relation to match paper

I4 based terms should only be included when I4 > 1, as fibres should
buckle and contribute no stiffness in compression.

This is mentioned in the paper but isn't actually made explicit in the
published equation.
Merge pull request #194 from adamreeve/holzapfel_ogden_fix

Fix Holzapfel-Ogden (2009) constitutive relation to match paper