activemerchant - updated econsultancy fork

By landing these commits we will:
2) Remove the activemerchant deprecation warnings
3) Have an updated suite of activemerchant tests, better reflecting the current datacash api
4) Be closer to synching our fork with shopify 

These commits are built on top of the latest [shopify activemerchant](https://github.com/Shopify/active_merchant) gem commits. We have some pull requests to have these landed onto [shopify activemerchant](https://github.com/Shopify/active_merchant/pulls) master. Currently 3 pull requests - when they have landed we can add the rest of our commits to a pull request for shopify. This doesn't hold up integration on our fork and can happen concurrently.

When these commits land we will switch activemerchant from a gitsubmodule to a gem in the deathstar.