Money gem deprecation warning tweaks

The calls to convert balance, buy, and sell prices to Money objects were tweaked to avoid generating deprecation warnings from the Money gem by using 'to_money'. There are plenty more warnings, but these are the first I hit playing with the coinbase gem.
![src__bash__11829](https://f.cloud.github.com/assets/41010/1501310/8c3970d6-4889-11e3-9d26-1bea4f97e42f.png)
![src__bash__11829-2](https://f.cloud.github.com/assets/41010/1501311/8ebc67a0-4889-11e3-92e9-9c78bf2ce220.png)