Upgrade pg gem to fix "truncated identifier" warnings.

Please see:
  https://bitbucket.org/ged/ruby-pg/issue/156
Add minitest gem so minitest/mock is available.
Rename memoizing variable for consistency with other models.
Add test database configuration.
Add unit tests for BiblioCommons and NYPL blog content items.
Add fixtures for NYPL blog content items, tweets, and URLs.