Fix readthedocs build (#721)

RTD updated their build environment which broke the 1.8.14.dev build of
doxygen that we were using. The update also breaks the conda-forge build
of 1.8.13 (but that version has other issues).

Luckily, the RTD update did bring their doxygen version up to 1.8.11
which is enough to parse the C++11 code we need (ref qualifiers) and it
also avoids the segfault found in 1.8.13.

Since we're using the native doxygen, conda isn't required anymore and
we can simplify the RTD configuration.

[skip ci]

.readthedocs.yml

-conda:
-  file: docs/environment.yml
+python:
+  version: 3
+requirements_file: docs/requirements.txt

docs/environment.yml

-name: rtd
-channels:
-- dean0x7d
-- defaults
-dependencies:
-- python==3.5
-- sphinx==1.5.1
-- sphinx_rtd_theme==0.1.9
-- doxygen
-- pip
-- pip:
-  - breathe

docs/requirements.txt

+breathe == 4.5.0