v1.6.0

Upgrade to Jackson 2.9.5
Upgrade to Caffeine 2.6.2
Upgrade to Netty 4.1.23.Final
Upgrade to Snake Yaml 1.20
ServerConfigBuilder.registerShutdownHook(boolean) defaults to true
HttpClient request/response interceptors
Expose HttpClient pool queue size
Promise.flatOp
Promise.mapError(Predicate,Function)
RatpackServer.getRegistry()
Fix for URL paths when serving files
Fix to not allow any empty string as the session id.
Configure HttpClient for Retrofit builders
Check for null content-type in retrofit request
ByteBuf metrics in DropwizardMetricsMetrics
Added rx2 Module
Added thymeleaf3 Module
Enhance RequestFixture with multipart form and file upload capabilities
Added reactor module