Fix: videojs.getComponent is not a function

aka videojs is not defined

Adds import video.js into each component, which eliminates the following error when
using video-http-source-selector with webpack:

```
videojs-http-source-selector.es.js:36 Uncaught TypeError: videojs.getComponent is not a function
    at Module.<anonymous> (videojs-http-source-selector.es.js:36)
    at Module../node_modules/videojs-http-source-selector/dist/videojs-http-source-selector.es.js (videojs-http-source-selector.es.js:264)
```

This change also resolves compiled output in dist/ mixing up use of `videojs` and `videojs$1`.

src/components/SourceMenuButton.js

@@ -1,3 +1,4 @@
+import videojs from 'video.js';
 import SourceMenuItem from './SourceMenuItem';
 
 const MenuButton = videojs.getComponent('MenuButton');

src/components/SourceMenuItem.js

@@ -1,3 +1,4 @@
+import videojs from 'video.js';
 const MenuItem = videojs.getComponent('MenuItem');
 
 class SourceMenuItem extends MenuItem