Try caching pip packages

.gitlab-ci.yml

@@ -3,13 +3,16 @@ image: python
 stages:
   - test
 
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/vendor/pip"
+
 cache:
   paths:
-    - vendor/apt
+    - vendor
     - .tox
 
 before_script:
-  - mkdir -p vendor/apt
+  - mkdir -p vendor/{apt,pip}
   # http://www.python-ldap.org/en/latest/installing.html
   - apt-get update -q && apt-get -o dir::cache::archives="vendor/apt" install -yqq build-essential python2.7-dev python3-dev libldap2-dev libsasl2-dev
   - pip install tox