Add the rustc version into dependency cache key

circle.yml

@@ -17,12 +17,12 @@ jobs:
           paths:
             - ~/.cargo/registry/index
       - restore_cache:
-          key: dependencies-{{ checksum "Cargo.lock" }}
+          key: dependencies-1.18-{{ checksum "Cargo.lock" }}
       - run: cargo test --all
       - run: cargo test --manifest-path=postgres/Cargo.toml --features "$(cargo read-manifest --manifest-path=postgres/Cargo.toml | jq -r '.features|keys|map(select(. != "with-security-framework" and . != "with-schannel"))|join(" ")')"
       - run: cargo test --manifest-path=tokio-postgres/Cargo.toml --all-features
       - save_cache:
-          key: dependencies-{{ checksum "Cargo.lock" }}
+          key: dependencies-1.18-{{ checksum "Cargo.lock" }}
           paths:
             - target
             - ~/.cargo/registry/cache