Fix browserify watch

ATM browserify bundles are never watched, not even for `grunt dev`, since@Kartones changed the config/grunt_development.json setting to false due to the (ENOSPC error)[http://stackoverflow.com/questions/16748737/grunt-watch-error-waiting-fatal-error-watch-enospc] on his linux box. 

This fixes it by only use watchify only for `grunt dev`, so the frontend task will work as before and won't interfere with non-frontend devs.