Disable interrupt option for Grunt watch

[kinglozzer]

If you save one file, then save a second one while the tasks are running, it’s interrupted but then it stops watching after it’s finished. Seems the spawn and interrupt options don’t always play nicely together. Played around with it, the spawn option speeds everything up quite a lot so I think that’s a more desirable feature than interrupt.

[stnvh]

It did seem a bit of a last ditch attempt to make grunt ever so slightly quicker! I don't think removing this will impact speed at all; The issue was when a PC is at high memory load, spawning another task is more difficult as it can't allocate memory as easily (hence why spawn: false)