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.

.jshintrc

@@ -0,0 +1,26 @@
+{
+	"boss": true,
+	"curly": true,
+	"eqeqeq": true,
+	"eqnull": true,
+	"es3": true,
+	"expr": true,
+	"immed": true,
+	"noarg": true,
+	"nonbsp": true,
+	"onevar": true,
+	"quotmark": "single",
+	"trailing": true,
+	"undef": true,
+	"unused": true,
+
+	"browser": true,
+
+	"globals": {
+		"_": false,
+		"Backbone": false,
+		"jQuery": false,
+		"JSON": false,
+		"wp": false
+	}
+}