v3.8.1

• Make TracingConfig an enum (see #234)
• Add CodeBuild cache (see #235)
• Add default None to SNS resources. (see #237)
• Change Events::Rule target Input to Token[String] (see #238)
• Add default values to optional EMR configurations (see #239)
• Fix IAM Policies (see #241)
• upgrade to sbt 1.1 (see #242)
• add VpcRef ADT to allow VPC ID to be set more flexibly (see #243)
• Add missing arguments to ApiGateway ApiKey (see #244)
• Update simple template documentation example so it compiles and runs (see #245)
• Upgrade/extend AWS::Elasticsearch::Domain to support VPCOptions (see #246)
• Add ApplicationAutoScaling (see #247)