Disable timeout by default for mocha 1.21.0 and above.

visionmedia/mocha@5dad9a34fe99fbfb4e3b72bfd6f780f4dcfed045 is included in 1.21.0.
On the commit, `lib/runnable.js` and `lib/suite.js` contains `this._enableTimeouts = true;` which breaks protractor, and it was reported on #1109.

If I add `enableTimeouts : false` to default mochaOpts, mocha no longer spits out "0s timeout" errors.