update changelog & bump version

Author: designatednerd

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Change log
 
+### v0.20.0
+
+- Fixed a bunch of data races in `ApolloWebSocket`. ([#880](https://github.com/apollographql/apollo-ios/pull/880))
+- Updated `ApolloWebSocket` to depend on `Apollo` in `Package.swift` since there is a dependency there. ([#906](https://github.com/apollographql/apollo-ios/pull/906))
+- **POSSIBLY BREAKING** Updated Swift tools verson in package declaration to 5.1. ([#883](https://github.com/apollographql/apollo-ios/pull/883))
+
 ### v0.19.0
 - **NEW**: Added a retry delegate to allow retries based on GraphQL errors returned from your server, not just network-level errors. NOTE: Be careful with which errors you retry for - the mere presence of an error doesn't necessarily indicate a full failure since GraphQL queries can return partial results. ([#770](https://github.com/apollographql/apollo-ios/pull/770))
 - **NEW**: Automatically generates ApolloEngine/ApolloGraphManager headers based on your main bundle's ID and version number. These can also be configured when you set up your `NetworkTransport` if you need something more granular for different versions of your application. ([#858](https://github.com/apollographql/apollo-ios/pull/858))

Configuration/Shared/Project-Version.xcconfig

@@ -1 +1 @@
-CURRENT_PROJECT_VERSION = 0.19.0
+CURRENT_PROJECT_VERSION = 0.20.0