Add an environment variable to bust the package

See here: pypa/pip#7217 (comment)

Basically, an internal change was made to Pip, but because the dependencies are cached, we're using a new verison of Pip with old dependencies which causes issues.

.circleci/config.yml

@@ -21,15 +21,15 @@ jobs:
             - last_image_versions
       - restore_cache:  # ensure this step occurs *before* installing dependencies
           name: restore dependency cache
-          key: deps-{{ .Branch }}-{{ checksum "Pipfile.lock" }}
+          key: deps-{{ .Environment.DEPS_CACHE_VERSION }}-{{ .Branch }}-{{ checksum "Pipfile.lock" }}
       - run:
           name: install dependencies
           command: |
             sudo pip install pipenv
             pipenv install
       - save_cache:
           name: save dependency cache
-          key: deps-{{ .Branch }}-{{ checksum "Pipfile.lock" }}
+          key: deps-{{ .Environment.DEPS_CACHE_VERSION }}-{{ .Branch }}-{{ checksum "Pipfile.lock" }}
           paths:
             - ".venv"
             - "/usr/local/bin"
@@ -62,4 +62,4 @@ workflows:
               only:
                 - master
     jobs:
-      - build
+      - build