Upgrade to kibana-rack 0.1.1

This upgrades kibana-rack and ensures that the dashboard endpoint
returns the correct content-type, which [the author says is an important
fix](https://twitter.com/tabolario/status/498831338011508736)

See [compare
url](https://github.com/tabolario/kibana-rack/compare/v0.1.0...v0.1.1)
Merge pull request #6 from alphagov/upgrade-kibana-rack

Upgrade to kibana-rack 0.1.1