Update Lambda runtime in sample YML for E2E tests

Author: aidansteele

sample/serverless.yml

@@ -4,7 +4,7 @@ Resources:
     Type: AWS::Serverless::Function
     Properties:
       Handler: index.handler
-      Runtime: nodejs8.10
+      Runtime: nodejs12.x
       MemorySize: 256
       CodeUri: ./func
       Timeout: 60