bump circle cache version

.circleci/config.yml

@@ -17,7 +17,7 @@ jobs:
       - checkout
 
       - restore_cache:
-          key: venv-3-{{ .Branch }}-{{ checksum "requirements.txt" }}
+          key: venv-4-{{ .Branch }}-{{ checksum "requirements.txt" }}
 
       - run:
           name: Base dependencies
@@ -48,7 +48,7 @@ jobs:
             if [ "$COVERALLS_REPO_TOKEN" != "" ]; then coveralls || true; fi
 
       - save_cache:
-          key: venv-3-{{ .Branch }}-{{ checksum "requirements.txt" }}
+          key: venv-4-{{ .Branch }}-{{ checksum "requirements.txt" }}
           paths:
             - /home/circleci/.pyenv
             # Ideally we'd cache these, but they need root, and the cimg/python