Fix Failing Build (gh-pages/doctr)

Travis CI is reporting build failing, appears to be related to making and pushing project Sphinx documentation to GitHub Pages using Python Doctr package.

```
ERROR: The doctr command '/home/travis/virtualenv/python3.5.10/bin/doctr deploy --built-docs docs/_build/html .' failed: SystemExit('\x1b[31mgit fetch doctr_remote failed: 128\x1b[0m',)
335git fetch doctr_remote failed: 128
```