Upgrade grunt-contrib-watch to ~0.5

I (and apparently other ppl) experienced some issues with grunt-contrib-watch and grunt 0.4 (basically the livereload is not happening anymore after a certain time); this 0.5 grunt-watch version seems to be way better. I also updated the nospawn to "spawn" in the Gruntfile since the spec changed (even if nospawn still works)

It's really only one commit but had to reset the other ones I made for the ngmin PR so they're not related