Unused history check

[aocenas]

Hi, just noticed that this check: https://github.com/yahoo/fluxible-router/blob/master/lib/handleHistory.js#L41, is never changed to true. So I assume, that what ever it should prevent (creating more history listeners I think), it does not do that right now. Did not really check with any code samples, but simple search for that variable never turns any assignment which would change it to true.

[mridgway]

Good catch. That was put in place so that there aren't multiple handleHistory higher-order components on the page at once... but it seems somewhere along the line it was broken. This won't break anything since it's more of a development warning, but I'll take a look.