Test duration custom relativeTimeThreshold

Please consider these new tests as a bug report. I've also created a fiddle here:

http://jsfiddle.net/ca9vfr3b/

It has to do with the rounding of the `duration.as('m')` value. When rounding down, 29 seconds becomes 0 minutes, so the logic here results in using the 'mm' unit instead of the 'm' unit:

https://github.com/moment/moment/blob/develop/moment.js#L1738

I attempted a fix by simply using Math.ceil() instead of round() but that turned out to be rather short-sighted as it resulted in failure of many other tests.