Remove nightly Python builds

Since they fail due to issues installing numpy.

See an example of this here:
https://travis-ci.org/github/ExcaliburZero/blender-astro-visualization/jobs/702930626

Author: ExcaliburZero

.travis.yml

@@ -4,7 +4,6 @@ python:
   - "3.7"
   - "3.8"
   - "3.8-dev"
-  - "nightly"
 
 install:
   - pip install coveralls