Updated wedge fix in symbols.py

Edited the wedge fix in `lib/iris/symbols.py`, adding a behaviour-based matplotlib version check to the function. The bug requiring the workaround this function supplies is fixed by matplotlib v1.3.x raising a `RuntimeError` if the workaround is attempted. To ensure backward compatibility with matplotlib v1.2.x this behaviour-based check is added.

Test image changed to remove the axes as these shift position in different versions of matplotlib and doing so allows the tests to pass in multiple matplotlib versions.

This is a preparatory PR for upgrading the Iris dependency to matplotlib v1.3.1.