Bump readthedocs-sphinx-ext to 0.5.5 to resolve search issues (#2834)
Bump readthedocs-sphinx-ext to 0.5.5 to resolve search issues (#2834)

Addresses #2708 by using a version of our extension that supports more than one
version of Sphinx.

Refs #1895
Refs #1850
Refs #2755
davidfischer/readthedocs.org,pombredanne/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,pombredanne/readthedocs.org,safwanrahman/readthedocs.org,pombredanne/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org