CI: port Travis config to GitLab

.gitlab-ci.yml

+image: python:2.7
+
+# Change pip's cache directory to be inside the project directory since we can
+# only cache local items.
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache"
+
+# Pip's cache doesn't store the python packages
+# https://pip.pypa.io/en/stable/reference/pip_install/#caching
+#
+# If you want to also cache the installed packages, you have to install
+# them in a virtualenv and cache it as well.
+cache:
+  paths:
+  - .cache/pip
+  - venv/
+
+before_script:
+  - python -V
+  - pip install virtualenv
+  - virtualenv venv
+  - source venv/bin/activate
+  - pip install ansible
+  - chmod 700 $(pwd)
+
+  - printf '[defaults]\nroles_path=../' > ./ansible.cfg
+
+lint:
+  stage: test
+  script:
+    - ansible-playbook tests/test.yml -i tests/inventory --syntax-check