Speed up watching

On my Linux, it's not using inotify for some reason -- perhaps some
issue with the gaze package (https://github.com/shama/gaze). We should
be using inotify eventually, but when we're not, we still want to be
fast. 20ms polling interval on a 100ms build step seems alright. (That's
100ms without spawning, which we can't disable quite yet though --
https://github.com/stefanpenner/ember-app-kit/pull/287#discussion_r7433842.)
Merge pull request #287 from joliss/watch-performance

Speed up watching