Merge branch 'release-2.2.0' into develop
Add turnout to enable a simple 'maintenance mode'

turnout should try to use JSON maintenance page by default, but it appears
that Rack won't do this unless the client specifies "Accept: application/json"
Travis: Force use of Elasticsearch 0.90.13 default config
Merge pull request #27 from dpla/feature/add-turnout

Add turnout to enable a simple 'maintenance mode'
Release v2.2.1
Merge branch 'release-v2.2.1'