Check screenshots on Travis using Docker and Selenium

Thanks to the docker service provided by Travis CI, we should be able to
download and use the Selenium docker images in order to run our
screenshotter and check whether all the screenshots match the images from
the repository.

.travis.yml

@@ -1,4 +1,13 @@
 language: node_js
 node_js:
-- "0.11"
-- "0.10"
+  - stable
+sudo: required
+services:
+  - docker
+before_script:
+  - docker pull selenium/standalone-firefox:2.46.0
+  - docker pull selenium/standalone-chrome:2.46.0
+script:
+  - npm test
+  - dockers/Screenshotter/screenshotter.sh
+  - git diff --name-only --exit-code