[chore] Fix Gruntfile to be compatible with Node 6.x
[chore] Fix Gruntfile to be compatible with Node 6.x

## Changes:

- Remove object spread operator since excaliburjs/excaliburjs.github.io still is on Node 6 👎 
excaliburjs/Excalibur,excaliburjs/Excalibur,excaliburjs/Excalibur