FLUID-4942: Fixes Infusion event system bug with Prototype.js.

This is a one line fix to fireToListeners, removing the for..in in favour of a regular for loop in order to avoid a breakage in uPortal due to Prototype.js.

Note that this bug is fixed in Infusion master, so this is just a quick fix for 1.4 users.

@amb26, @michelled or @jobara, can you review this and push it to the infusion-1.4.x branch?