Update dependencies, cleanup

[Firehed]

The next version of the library will require PHP 7.2, so this pulls in newer versions of dev dependencies and updates the code and tests to be compatible with them.

Notably:

• Numerous new PHPStan errors were caught and fixed (mostly json_decode related)
• PHPUnit setUp is now explicitly void
• The questionable workarounds to mbstring.func_overload > 0 have been removed, as the directive has been deprecated in PHP. This library is now straight-up incompatible with the setting, and will throw an exception if the setting is detected.

So far this makes to actual breaking changes in sanely-configured environments.

[coveralls]

Coverage increased (+1.1%) to 99.265% when pulling 3d0b22815ae53eb300d78f03b9479bdfe0564b49 on dependencies into 65bb799b1f709192ff7fed1af814ca43b03a64c7 on master.