Update prismic

[richardstevens]

What does this PR do? (please provide any background)

Updating metalsmith-prismic to clear old hardcoded dependancy and fix for new version of prismic api

How can this be tested?

locally npm link this repo to a ssg-sites repo, then run as normal

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[elliottcrush]

Great stuff, nice trimming of dependencies as well. Have tried this out locally with static-site-generator-sites on the testing branch and it works well. Glad that we've got more support for different data stores inside prismic. 👍