Add environment preference for Braintree

This allows you to specify what environment (sandbox or production) you would like the gateway to use.

Fixes this build: http://ci.spreecommerce.com/viewLog.html?buildId=5911&buildTypeId=bt14&tab=buildLog
Use BraintreeBlueGateway for Braintree gateway

This is due to the following code in ActiveMerchant: https://github.com/Shopify/active_merchant/blob/7678a0319a8a6234a793e8276eeb83c16b9e6750/lib/active_merchant/billing/gateways/braintree.rb#L10-L15

Since our gateway doesn't even support the login option, then it's obvious that we'll only be using blue
[braintree] No need to pass additional parameters in authorize test