Coding Standards: Move the remaining PHPCS errors to report as warnin…

…gs, and add Travis tests.

The remaining error-level coding standards issues (specifically, associated with the sniffs `WordPress.PHP.YodaConditions.NotYoda`, `WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase`, `WordPress.DB.PreparedSQL.InterpolatedNotPrepared`, `WordPress.DB.PreparedSQL.NotPrepared`, and `WordPress.Files.FileName.InvalidClassFileName`) are marked as warnings, until they're all addressed.

This change allows us to run linting on Travis across the entire codebase, ensuring no other error-level violations can be introduced.

Additionally, PHPCS will now cache results locally, drastically improving performance for subsequent checks: scanning the entire codebase takes 1-2 minutes the first time, and less than one second for subsequent checks.

See #47632.


Built from https://develop.svn.wordpress.org/trunk@45665


git-svn-id: http://core.svn.wordpress.org/trunk@45476 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45664';
+$wp_version = '5.3-alpha-45665';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.