Confirm PHP8.1 support

Minor release because there were no changes required and I'm not removed support for PHP7.4

Changed

• Confirmed everything works as expected on PHP 8.1.

Removed

• Removed an old dependency from when I used Travis

Single item lists never close

Fixed

• Fixed an issue rendering single item lists, they don't get closed if they are the first content, thank you hybridvision.
• Minor code style fixes.
• Corrections to README.

PHP 8 and 7.4

• Update requirements to PHP 8 or PHP 7.4.
• Update tests to run on latest version of PHP Unit.
• Removed Travis config, will switch to GitHub actions at some point soon.

Single item lists never close

Fixed

• Fixed an issue rendering single item lists, they don't get closed if they are the first content, thank you hybridvision.
• Minor code style fixes.
• Corrections to README.

Custom attributes on videos

• Videos with custom attributes supported, thank you Nicholas Humphries.

Define custom attributes to ignore

• Custom attributes values which aren't strings are ignored.
• Custom attributes which the parser should ignore can now be defined.

Allow arrays as well as json

• Allow decoded json to be passed in rather than the library decode it, thank you Lode Claassen.
• Validation code DRY, thank you Lode Claassen.

Exception messages incorrect

• Missing supported format in exception methods, thank you Lode Claassen.

Allow through all deltas

• Additional work to fix #117, allow through any empty insert, some are valid so for now just render what quill wants, I'll add strict mode later to remove what I consider to be the redundant inserts.

Compound deltas not aware they can be links

• Fixed #117, compound deltas not aware of the fact that they can also be links.
• Fixed #109 again as it appears I did not fix it correctly before.