Fix grunt dependencies for npm 2.0.0

This should resolve the install issue opened at mozilla/recroom#41 Though there are still significant issues with npm 2.0.0 that are being worked out. (https://github.com/npm/npm/issues/6043 / https://twitter.com/othiym23/status/507042794997682176)

Everything should still work as expected when using with npm 1.4.26 but would like somebody else to test out as well.