Fix #1246. Captions start language is not ticked in Firefox

See #1246. Also, test the following in Firefox and in other browsers: http://jsfiddle.net/mE3xb/2/
Firefox has some weird and inconsistent behavior with jquery .click()
Changing in src/js/mep-feature-tracks.js
```
$('#' + t.id + '_captions_' + lang).click();
```
to
```
$('#' + t.id + '_captions_' + lang).prop('checked', true).trigger('click');
```
fixes this and should work for all browsers too.