Bugfix: s13sitecore.ContentModelTests.test_get_article_siblings

This test failed because we didn't update it when we modified the
Article.get_siblings method to include the Article instance. Now,
the test works and we're happy.
Bugfixes: 2 tests in s13sitecore.FMSTests

* test_set_directory_success
    This test used to fail because the directory couldn't created
    since the media directory didn't exist (in s13sitedemo). The
    solution was to create a file inside it (now directory.info)
    so that Git would track the directory; hence, when s13 is
    cloned and this test is run, the test directory can be made.

* test_view_file
    This test used to fail because at some point, we modified the
    template to reflect a message that the test method was not
    expecting.