remove travis configuration and update readme build status (#171)

Author: AnderEnder

README.md

@@ -1,5 +1,7 @@
 # Rust Runtime for AWS Lambda
 
+[![Build Status](https://github.com/awslabs/aws-lambda-rust-runtime/workflows/Rust/badge.svg)](https://github.com/awslabs/aws-lambda-rust-runtime/actions)
+
 This package makes it easy to run AWS Lambda Functions written in Rust. This workspace includes multiple crates:
 
 - [![Docs](https://docs.rs/lambda_runtime_client/badge.svg)](https://docs.rs/lambda_runtime_client) **`lambda-runtime-client`** is a client SDK for the Lambda Runtime APIs. You probably don't need to use this crate directly!