Bump version to 1.6.0

[kstich]

1.6.0 (2020-02-22)

Features

• Added support for checking backwards compatibility for diffs of trait contents. (#716)
• Added support for adding CORS configurations to API Gateway HTTP APIs. (#670)
• Relaxed constraints on the @httpPayload trait, allowing it to target list, set, and map shapes. (#679)
  • Targeting these shapes is invalid for operations utilizing the aws.protocols traits. (#679,
    #683)
• Added validation to ensure a payloadFormatVersion is set when generating an API Gateway HTTP API. (#688)
• Added vendorParamsShape to protocol test cases to support validating a test case's vendorParams values are
  configured properly. (#702)
• Added the ability to validate resolved hosts to protocol tests. (#707)
• Added backwards compatibility checking to smithy-diff for the @paginated trait. (#716)
• Added tags and appliesTo to protocol test definitions for better categorization and grouping. (#696)
• Added several protocol tests for the endpoint and hostLabel traits. (#708)
• Added several @restXml protocol tests. (#689, #690,
  #678, #694)

Bug Fixes

• Fixed an issue that produced duplicate entries in the security list of a converted OpenAPI document. (#687)
• Fixed an issue where alphanumericOnlyRefs was not fully satisfied when generating synthesized shapes. (#695)
• Fixed several issues in IDL parsing where duplicate bindings were allowed incorrectly. (#714)
• Fixed several issues in protocol tests around serialization of empty contents. (#692)
• Fixed an issue where parameters in a diff error message were swapped. (#713)
• Fixed an issue in a restXml protocol test. (#715)

AWS Customizations

• Added a configuration definition for use validating vendorParams in AWS protocol tests. (#705)
• Added tests and documentation for some required Amazon S3 customizations. (#709)
• Added tests and documentation for required Amazon Glacier customizations. (#704)
• Added a test and documentation for the required Amazon API Gateway customization. (#706)
• Added a test and documentation for the required Amazon Machine Learning customization. (#707)

Documentation

• Improved the documentation for the awsJson1_0 and awsJson1_1 protocols. (#698)
• Improved the documentation for the awsQuery and ec2Query protocols. (#700)
• Clarified that Smithy requires support for fractional seconds for the http-date value of @timestampFormat. (#672)
• Added missing shape documentation for some waiters related shapes. (#711)
• Fixed several minor documentation issues. (#681, #693,
  #697, #701, #708,
  #717)

Other

• Migrated to using Gradle 6 to build Smithy. This should have no impactful downstream effects. (#194)
• Migrated to using main from master for the default branch. This should have no impactful downstream effects. (#685)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.