Merge branch 'update-gitlab-ci' into 'master'

Update GitLab CI configuration

*Description of changes*

Update GitLab CI. After reading [GitLab's blog post](https://about.gitlab.com/2016/07/29/the-basics-of-gitlab-ci/) about their CI system, it seems that we can easily start generating build artifacts for PRs and regular pushes.

*Related to:*  N/A

See merge request !111

.gitlab-ci.yml

@@ -1,22 +1,49 @@
+image: python
+
+stages:
+- test
+- build
+- release
+
 before_script:
-  - apt-get update -qq
-  - apt-get install -y python python3 python-dev python3-dev
-  - curl -O https://bootstrap.pypa.io/get-pip.py
-  - python get-pip.py
-  - pip install -r dev-requirements.txt
+- curl -O https://bootstrap.pypa.io/get-pip.py
+- python get-pip.py
+- pip install -r dev-requirements.txt
 
 python2:
-  script:
-    - tox -e py27
+  image: python:2.7
+  stage: test
+  script: tox -e py27
 
 python3:
-  script:
-    - tox --skip-missing-interpreters -e py34,py35
+  stage: test
+  script: tox --skip-missing-interpreters -e py34,py35
 
 linters:
-  script:
-    - tox -e linters
+  stage: test
+  script: tox -e linters
 
 docs:
-  script:
-    - tox -e docs
+  stage: test
+  script: tox -e docs
+
+dogfood:
+  stage: test
+  script: tox -e dogfood
+
+build:
+  stage: build
+  script: tox -e build
+  artifacts:
+    paths:
+    - dist/
+    expire_in: 1 month
+
+release:
+  stage: release
+  script: tox -e build
+  artifacts:
+    paths:
+    - dist
+  only:
+  - tags