Force Jekyll 1.5.1

This commit forces Jekyll 1.5.1 to deploy the site. Different versions of Jekyll render the site with small differences, and that makes almost every commit to the SVN site repository contain many files that are unrelated to the change being committed.

To avoid this, we should make sure we all generate the site using the same version of Jekyll when deploying it. This commit forces version 1.5.1, which is the one used by the CI system to validate the pull requests,