Update dependencies and tag for 5.2 compatibility

[adamsilverstein]

Description of the Change

• Update dependencies in package.json and composer.json to current versions.
• Fix some PHP notices w/PHP 5.6 and fatals with PHP 7.2/3.

Fixes #19

Verification Process

• Test with WordPress 5.2 and WordPress trunk.

Test with PHP 5.6 & 7.2.

Verify works as expected:

• Print Issues: create, add articles, change article statuses, Export for InDesign: all and checked.
• Issue Statuses, Publications, Article Statuses: create and use.
• Test Gutenberg compatibility: no changes are made to the post editor with this plugin so far. The "Print" custom post type added by the plugin opens in the classic edit environment because show_in_rest is not set (Gutenberg requires REST API access to use a CPT). Everything works as expected in my testing.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

[jeffpaul]

looks good to me, defer to @helen on final approval

[helen]

I wanted to manually review what's changed here, I think it's fine (although a little odd IMO) but just wanted to note things in case it becomes a problem later:

1. Rather than grouping a bunch of selectors for the same usage of a given property (border, display), the new build seems to prefer to keep all properties for a selector together.
2. In #publishing-actions, it goes from padding: 8px 9pt to padding: 8px 12px. The Sass file has 8px 12px so that's a fix IMO.
3. .pi-article-results now keeps both display: block and display: none as in the source - I actually cannot figure out where pi-article-results is used or generated at the moment, so that probably deserves investigation at some point. It may be that it's a default-inline element being made a block element and hidden/shown using display so it kind of makes sense, but it's also weird and would have been broken before. We may not need this ruleset at all.