Tests: run against the *new* PHPCS 4 branch

The PHP_CodeSniffer 4.0 branch has been completely rebuild. See PHPCSStandards/PHP_CodeSniffer#120 for more detailed information about the "why".

In practice, this means that to test against the upcoming PHPCS 4.0 release, we need to test against the `4.x` branch now, not the old `4.0` branch.

A 4.0 beta/RC release is expected in the next few days. Once that is out, a test period starts and depending on whether problems are found or not, the 4.0.0 release will be tagged about a month later (or later if significant problems were found).

When 4.0.0 gets tagged, the PHPCS `master` branch will be renamed to `3.x` and the `4.x` branch will be the new "main" branch for PHPCS, so more changes will be needed, but that's for later.

Author: jrfnl

tests/PHPCSVersions.php

@@ -27,7 +27,7 @@ final class PHPCSVersions
      *
      * @var string
      */
-    const NEXT_MAJOR = '4.0.x-dev as 3.9.99';
+    const NEXT_MAJOR = '4.x-dev as 3.9.99';
 
     /**
      * List of all PHPCS version which are supported by this plugin.