Remove sprockets and active resource dependencies

[jduff]

What this PR does

Removes sprockets and activeresource dependencies. If people need these they can add them to their own app.

fixes #1936

Reviewer's guide to testing

Test with a new Rails app.

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.