Unpin Sphinx and require RTD theme >= 0.4.2

Description

The HTML search problem with RTD theme + Sphinx 1.8 has been fixed upstream.

Changes proposed in this merge request:

• Unpin the sphinx requirement.
• Require sphinx-rtd-theme >= 0.4.2.

This merge request, when merged, will address issue/bug:

#114 (closed)

approved this merge request

merged

mentioned in issue #114 (closed)