Remove server and test mode parameters from Braintree
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
[braintree] Default environment preference to :sandbox if not set
Honor 'auto_run_migrations' in InstallGenerator

Fixes #74

Conflicts:
	lib/generators/spree_gateway/install/install_generator.rb