TRAPI 1.5 support and enhanced TRAPI version resolution

[RichardBruskiewich]

• [ ] Ensure full validation of TRAPI 1.5 (including beta's) including appropriate unit tests
• [ ] If the user specifies None for the schema version, just use the latest version that the code knows about. But if the user specifies a version that the code doesn't know about, it should halt with an informative error message. Even better would be to check GH to see if that version really exists, and if it does, either pull it down or send request to @Richard Bruskiewich (SRI) that a Slack message that the code needs updating!