These changes makes zuul use ngrok for exposing itself to the outside world, rather than the default localtunnel implementation. That's because we've had quite a lot of issues with flakyness which seems to be related to localtunnel, e.g.:
```
- starting: <internet explorer 9 on Windows 2008>
events.js:160
throw er; // Unhandled 'error' event
^
Error: connection refused: localtunnel.me:44896 (check your firewall settings)
at Socket.<anonymous> (/home/travis/build/janl/mustache.js/node_modules/localtunnel/client.js:84:32)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at emitErrorNT (net.js:1290:8)
at _combinedTickCallback (internal/process/next_tick.js:80:11)
at process._tickCallback (internal/process/next_tick.js:104:9)
```
I thought this was fixed by [using Node.js 6 instead of Node.js 4](https://github.com/janl/mustache.js/pull/667), but sadly that was not the case as it re-appeared with Node.js 6 as well.
Had to use a [custom version of zuul-ngrok](https://github.com/rase-/zuul-ngrok/pull/12) which contains a newer version of ngrok. The dependency used by the upstream module threw an error when running on my computer, the updated version worked as a charm though. We should change to using the upstream module as soon as the PR with that ngrok version bump lands.
Set `concurrency: 1` otherwise ngrok would fail and make IE11 tests stall forever because of too many connections being made within a short time period. I've notice it can take quite some time (20+ minutes) to get all the tests through as it seems ngrok still seems to throttle the connections somehow, but the important thing is getting these tests to work.
With 25 different browser versions it could easily take 15-25 minutes for a run too finish.
Only test latest chrome & firefox atm, increase support later as needed.
Covers the project with browser tests, not only V8 via node/iojs. Tests are runned with [zuul](https://github.com/defunctzombie/zuul) which
provides simple local browser testing, aswell as cloud testing on multiple browsers with [saucelabs](http://saucelabs.com).