Do not require inbound parser to be installed

Version Bump v6.0.0: BREAKING CHANGE

Version Bump v5.6.0: Dynamic Template support

Version Bump v5.5.0: Pre-Dynamic Template Roll Up Release

Version Bump v5.4.1: Bug Fixes

Version Bump v5.4.0: Hacktoberfest Release

Version Bump v5.3.0: sendgrid#348: Allows users to submit rfc822 form…

…atted email addresses

Version Bump v5.2.1: Pull sendgrid#364: Install prism with non superu…

…ser account

Version Bump v5.2.0: sendgrid#335: Permit unicode string values with …

…Substitution helper

Version Bump v5.1.0: Pull sendgrid#338, Solves sendgrid#292 __str__