ensure docs build properly on readthedocs

[satra]

nipype on readthedocs has not been building properly, since release 1.1.1:

https://readthedocs.org/projects/nipype/builds/7835987/

we should try to ensure that our own doc build returns appropriate errors on circle if they fail, which i thought it did. we should resolve what it is between our tests and readthedocs is altering the failure mode.

according to the error above it seems something about matplotlib sphinx extension

[satra]

this extension exists at least in matplotlib release 2.2.2, which is available inside the nipype/nipype:py36 docker container.

so this is likely some setting in/for readthedocs that's causing issues.

[miykael]

I tried to build the docs locally and got an error concerning matplotlib.sphinxext.only_directives. The easy solution was to just remove/comment this dependency in the conf.py file (here). But I'm not sure if it will lead to other errors down the line (or for what this extension is actually needed).