Minor docs build maintenance (#692)

* 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]

docs/environment.yml

@@ -4,7 +4,9 @@ channels:
 - defaults
 dependencies:
 - python==3.5
+- sphinx==1.5.1
+- sphinx_rtd_theme==0.1.9
 - doxygen
 - pip
 - pip:
-  - https://github.com/michaeljones/breathe/archive/master.zip
+  - breathe