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.

.jscsrc

@@ -0,0 +1,7 @@
+{
+    "preset": "wordpress",
+    "fileExtensions": [ ".js" ],
+    "excludeFiles": [
+    	"js/skip-link-focus-fix.js"
+    ]
+}