Fix addEventListener for older version Firefox

I kept getting these errors in Firefox 3.6.25:

uncaught exception: [Exception... "Not enough arguments" nsresult: "0x80570001 (NS_ERROR_XPC_NOT_ENOUGH_ARGS)" location: "JS frame :: http://[removed]/js/pace.js :: anonymous :: line 570" data: no]

The third argument for `addEventListener` and `removeEventListener`
is supposed to be optional, defaulting to `false`, but some browsers
(notably older versions of FireFox) require it.

https://developer.mozilla.org/en-US/docs/Web/API/EventTarget.removeEventListener