Braintree: Allow dynamic descriptors

Docs for it:
https://www.braintreepayments.com/docs/ruby/transactions/dynamic_descriptors

Braintree added the ability to also specify a descriptor url here:
https://github.com/braintree/braintree_ruby/commit/7a336acb9eecc4293e05af2c43a8f15848cf6f55#diff-bf222cdd67b742610b54bdf5774362fc

but Active Merchant is using an older version of the Braintree gem.  Not
sure if we want to upgrade to a more recent version.