Fixes issue when Mocha.settings is not available

I turned off the `insecure` and `autopublish` packages in Meteor, and the `mocha-web` package stopped working. Specifically, it gave the error:

    Uncaught TypeError: Cannot read property 'public' of undefined 

On line 2 of `preTest.js`. I believe this is because the settings object is not published by default, unless one were to opt-in by [specifying some public settings](http://docs.meteor.com/#meteor_settings) to be available to the client.

In my fix, I test that the Mocha.settings object exists before preceding.