Use ansible-lint==2.3.6

Before sudo rule merge
https://github.com/willthames/ansible-lint/pull/96

* Add test-requirements.txt
* travis: enable cache for pip packages

.travis.yml

 language: python
 python: "2.7"
+cache:
+  directories:
+    - $HOME/.cache/pip
 install:
-  - pip install ansible ansible-lint
+  # TODO: use requirements.txt when ansible updated to >= 1.9
+  - pip install ansible
+  - pip install -r test-requirements.txt
 script:
   - ansible-playbook --syntax-check -i hosts site.yml
   - ansible-lint site.yml

requirements.txt

 ansible==1.6.6
-

test-requirements.txt

+ansible-lint==2.3.6