Hotfix to build script

The pytest version was not compatible with `pytest-cov` and the sphinx version clashed with a plugin.

.travis.yml

@@ -6,8 +6,8 @@ language: python
 python:
   - "3.6"
 install:
-  - pip install pytest pytest-cov # testing 
-  - pip install sphinx # python documentation tool
+  - pip install 'pytest>=4.6' pytest-cov # testing 
+  - pip install 'sphinx<=2.4.4' --force-reinstall # python documentation tool
   - pip install sphinx_bootstrap_theme # documentation theme
   - pip install sphinx_automodapi # automated function documentation
   - pip install nbsphinx # jupyter notebook documentation support