Added compatibility with Money 6.x.

The Money gem has deprecated parsing in 6.0. However, in 6.1 they are bringing conversions back. This is just a copy paste of the old money parsing code, which makes this gem compatible with all versions.

You should cut a release with this change.

I also have a [branch](https://github.com/dblock/money_helper/tree/money-6.1), which I will PR once Money 6.1 is released.