Merge branch 'feature/pdf-export' into 'master'

Remove broken pdf export options

See merge request jupyter/clb-jupyter-nb-images!8

.gitlab-ci.yml

 variables:
   docker_registry_user: "robot$$gitlab"
-  docker_image_tag: "1.4"
+  docker_image_tag: "1.5"
 
 stages:
   - build

base/jupyter_notebook_config.py

@@ -7,6 +7,9 @@ c.NotebookApp.port = port
 c.NotebookApp.open_browser = False
 c.NotebookApp.quit_button = False
 
+c.PDFExporter.enabled = False
+c.WebPDFExporter.enabled = False
+
 if os.environ.get('JUPYTERHUB_SERVICE_PREFIX'):
     c.NotebookApp.base_url = os.environ.get('JUPYTERHUB_SERVICE_PREFIX')