Use page version's `updated_at` timestamp as cache key

[mamhoff]

What is this pull request for?

Currently, it's hard for us to invalidate caches for Alchemy pages when content that's referenced through ingredients with related objects changes.

For example, in a situation where a user combines Alchemy and Solidus using the alchemy_solidus gem, a page's cache key does not update when a product that's referenced through a SpreeProduct ingredient changes.

There's a PR up on the alchemy_solidus gem that touches ingredients in these situations, and with this change, that touching can be used for breaking caches. [1]

1

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change