Build/Test Tools: Update `.travis.yml` to include latest improvements from trunk.

* Explicitly use PHPUnit 5.7 for the PHP 7 builds on Travis.
* On Travis CI install and use the node version which is specified in package.json.
* Add some more debugging to Travis and bring the format of the Xdebug fix inline with branches.

Merge of [40255] and [40257-40259] to the 4.7 branch.

Props netweb, johnbillion.
See #35105, #39822, #40086.
Built from https://develop.svn.wordpress.org/branches/4.7@40260


git-svn-id: http://core.svn.wordpress.org/branches/4.7@40182 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7.4-alpha-40253';
+$wp_version = '4.7.4-alpha-40260';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.