Expose only the specified UI in the browser

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).
Merge pull request #984 from jugglinmike/browser-ui

Expose only the specified UI in the browser

* jugglinmike/browser-ui:
  Expose only the specified UI in the browser