fix: use es6 class inheritance

The warning below is appearing in the browser console:

```
VIDEOJS: WARN: videojs.extend is deprecated as of Video.js 7.22.0 and will be removed in Video.js 8.0.0
```

Issue silvermine#152

tests/ChromcastButton.test.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var expect = require('expect.js');
+
+const chromecastButton = require('../src/js/components/ChromecastButton');
+
+describe('ChromecastButton', function() {
+   it('should not call videojs.extend', function() {
+      const videoJsSpy = {
+         extend: function() {
+            expect().fail('videojs.extends is deprecated');
+         },
+         getComponent: function() {
+            // no op
+         },
+         registerComponent: function() {
+            // no op
+         },
+      };
+
+      chromecastButton(videoJsSpy);
+      expect().to.not.fail();
+   });
+});

tests/ChromcastTech.test.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var expect = require('expect.js');
+
+const chromecastTech = require('../src/js/tech/ChromecastTech');
+
+describe('ChromecastTech', function() {
+   it('should not call videojs.extend', function() {
+      const videoJsSpy = {
+         extend: function() {
+            expect().fail('videojs.extends is deprecated');
+         },
+         getComponent: function() {
+            // no op
+         },
+         registerComponent: function() {
+            // no op
+         },
+      };
+
+      chromecastTech(videoJsSpy);
+      expect().to.not.fail();
+   });
+});