Revert "Update json from 1.8.0 to 1.8.1"

This reverts commit 8d7ae2da6f74389d8d1107cde3f7df6092e3e0ea.

We still have to upgrade the gem, but it'll be after the site is migrated
to Ruby 2.1.0 final, or probably to Ruby 2.1.1 when it is released.