Preparing for v6.0

[dbhynds]

I'm prepping to release a v6.0 of this library.

• Read a summary / upgrade guide here: https://github.com/packbackbooks/lti-1-3-php-library/blob/v6.0-beta/UPGRADES.md#5x-to-60
• You can see all changes here: v6.0 beta #127

We've been using this branch in production for a couple weeks as the spring semester started, and haven't had any issues. That said we don't use all of the functionality that this library offers in our app.

If anyone feels like helping beta test this, I'd love to hear if you run into any issues. I'd also love feedback on the upgrade guide and process. Thank you!

[dbhynds]

@erikdonohoo if you have time, I'd love your feedback (whether here or on the PR above).