Use specific python and package versions in CI script

The automatic generation of the documentation with GitLab CI stopped
working because installing h5py with pip caused errors. Using the
correct versions of python, numpy and h5py fixed this.

.gitlab-ci.yml

 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: python:latest
+image: python:3.6.8
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.
@@ -18,17 +18,19 @@ cache:
     - venv/
 
 before_script:
-  - python -V  # Print out python version for debugging
+  - python --version  # for debugging
+  - pip --version     # for debugging
   - pip install virtualenv
   - virtualenv venv
   - source venv/bin/activate
+  - pip install sphinx
+  - pip install numpy==1.16.2
+  - pip install h5py==2.8.0
+  - pip install configparser
+  - pip list          # for debugging
 
 pages:
   script:
-    - pip install sphinx
-    - pip install numpy
-    - pip install h5py
-    - pip install configparser
     - cd doc
     - make html
     - mv build/html/ ../public/