Add Rake tasks to clear limiters

When many integration tests are run in a short period, the local bitpay server
will refuse to create new keys due to the rate limiting function. This
rake task clears the ratelimiters database.
Bump required ruby version for gem

Ruby 2.0 does not support required keyword arguments, but we would like
to use required keyword arguments.
Use the public interface to retrieve an invoice

With this commit the following functionality is complete:
1. Using a token, pair a key
2. Create a new invoice
3. Retrieve the public invoice