package.json should include compute-cluster in dependencies

Include compute-cluster in "dependencies" so `npm install --production`
works for building assets.

Test plan:
  - run rm -rf node_modules && npm install --production
  - run bundle exec rake canvas:compile_assets

Change-Id: Ie5b1d387608f24b59a736dbeb974cb1ec710cfbd
Reviewed-on: https://gerrit.instructure.com/34302
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>
QA-Review: Stanley Stuart <stanley@instructure.com>