Update History.js, error el is not defined

I assume that "el" was meant to be "target" in this instance? "el" does not exist in the function.
Merge pull request #10 from safani/patch-1

Update History.js, Error el is not defined
Fixed mounting Derby app to an Express app using route other then '/'

Without this fix routing will work only on server side, because request
would be handled by Express first before passing it to `tracks` router.

Client side routing will fail because `tracks` router will receive
prefixed URL for handling, e.g. it will receive '/derby-app/home' while
knowing only about '/home' route.