Slim down the travis script, add js checks and fix js errors thrown up.

What it will now do:
- Lint the php files against relevant PHP versions.
- Lint the js files once - the result won't change across PHP versions.
- Check against WPCS once - the result won't change across PHP versions.

What I have changed:
- Added linting against PHP 7 and HHVM, with HHVM being allowed to fail.
- Added js linting and style check per example from Twenty Sixteen.
- Moved to the faster container based environment for running travis.
- Script will no longer pull in PHPCS, WPCS and the JS linters in every build. Now they will only be pulled in when needed.
- Limited the clone depth for quicker cloning of external repositories.
- Removed the pulling in of WP and running builds against different WP versions as this wasn't used at all in the actual test scripts.

js/navigation.js

@@ -1,11 +1,11 @@
 /**
- * navigation.js
+ * File navigation.js.
  *
  * Handles toggling the navigation menu for small screens and enables tab
  * support for dropdown menus.
  */
 ( function() {
-	var container, button, menu, links, subMenus;
+	var container, button, menu, links, subMenus, i, len;
 
 	container = document.getElementById( 'site-navigation' );
 	if ( ! container ) {
@@ -47,7 +47,7 @@
 	subMenus = menu.getElementsByTagName( 'ul' );
 
 	// Set menu items with submenus to aria-haspopup="true".
-	for ( var i = 0, len = subMenus.length; i < len; i++ ) {
+	for ( i = 0, len = subMenus.length; i < len; i++ ) {
 		subMenus[i].parentNode.setAttribute( 'aria-haspopup', 'true' );
 	}