* Switch breathe to stable releases. It was previously pulling directly
from master because a required bugfix was not in a stable release yet.
* Force update sphinx and RTD theme. When using conda, readthedocs pins
sphinx==1.3.5 and sphinx_rtd_theme==0.1.7, which is a bit older than
the ones used in the RTD regular (non-conda) build. The newer theme
has nicer sidebar navigation (4-level depth vs. only 2-level on the
older version). Note that the python==3.5 requirement must stay
because RTD still installs the older sphinx at one point which isn't
available with Python 3.6.
[skip ci]
