Expose only the specified UI in the browser

I'm happy to add tests for this, but it doesn't look like there are any tests for the browser build--is this the case?

Commit message:

> When instantiated in the browser, Mocha registers the "BDD-style"
> interface. Since an explicit call to the browser-specific `mocha.ui` is
> required, the original registration is invalid. Prevent that UI from
> reacting to the `pre-require` event (and exposing its API).