2.2.18

Version 2.2.18 February 11, 2016

• Add mark_failed API call to Invoice
• Add the python version to the user agent string
• Fix the delete_url on the Redemption class by removing the override
• Add support for cc_emails attribute in Account class
• Remove AttributeError when trying to print ValidationError in Python 3
• Remove _ValidatedHTTPSConnection in favor of native functionality
• Add collection_method and net_terms attributes for invoice calls