Build Tools: Switch all Travis PHPUnit jobs over to Docker-based PHP.

Additionally, there are a handful of related minor changes in this commit:
- `.travis.yml` has been tidied up a little.
- [45745] was incorrectly marking Travis jobs as passed when some PHPUnit runs failed.
- Add the `LOCAL_PHP_MEMCACHED` environment variable, for enabling Memcached.
- Add the `env:pull` NPM script, for refreshing Docker images.

See #47767.

Built from https://develop.svn.wordpress.org/trunk@45762


git-svn-id: http://core.svn.wordpress.org/trunk@45573 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45761';
+$wp_version = '5.3-alpha-45762';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.