Slim 3.12.0 released

It's a new year, so a new Slim! Like 3.11.0, version 3.12.0 is nearly entirely bug fixes.

In particular, #2543 & #2558 enable Slim to be used with Diactoros which requires read() to be called with an integer.

Also, #2519 adds getBasePath() to the Router object.

The full list of changes is here