Bumping Circle to use Rust 1.42.0

subtle was failing on 1.40, then postgres-protocol was failing on 1.41.

Author: dpetersen

.circleci/config.yml

@@ -26,7 +26,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: rust:1.40.0
+      - image: rust:1.42.0
       - image: postgres:12
         environment:
           POSTGRES_PASSWORD: password