Prepare "coming soon" update for front page

This is a small placeholder that will appear on the public Hub at
https://18f.gsa.gov/hub/ before launch.
Tests for redact! and publish_snippet()

One of the publish_snippet() tests also promoted a bug fix, whereby any
missing "last-week" or "this-week" field is now assigned the empty string to
avoid a nil dereference at the end of the function.
Publish only v3 snippets in public mode

Also adds a complete test for join_snippet_data.
Minor tweak to join_snippet_data per @afeld in #16
Merge pull request #16 from 18F/public-snippets-v3-only

Only publish v3 snippets in public mode