fix minified videojs in ie8

[gkatsev]

The issue here is that babel was turning function expressions like let Foo = function() {}
in names function expressions like var Foo = function Foo() {}. Afterwards, the minifier came through and cause these two variables to diverge causing issues in IE8. The fix is to switch to function statements.
Fixes #3064 and #3070.

[gkatsev]

This is only a problem because of the special handling we do for tracks in IE8, specifically:

video.js/src/js/tracks/text-track.js

Lines 44 to 52 in 084bcf6

	if (browser.IS_IE8) {
	tt = document.createElement('custom');
	for (let prop in TextTrack.prototype) {
	if (prop !== 'constructor') {
	tt[prop] = TextTrack.prototype[prop];
	}
	}
	}

[misteroneill]

LGTM then