Fix RTD builds broken by the notebook header (#401)

It's not clear what broke this or whether it was ever supposed to be used the way we had it before

`current_version` and `commit` are defined by https://github.com/readthedocs/readthedocs.org/blob/bc3e147770e5740314a8e8c33fec5d111c850498/readthedocs/doc_builder/backends/sphinx.py#L163-L205.

I can't work out where `READTHEDOCS` comes from, but `sphinx_rtd_theme` seems to use it like this.

This fixes the builds, although it might make the notebook links always use `master` which would be not ideal. I guess we'll find out.

docs/conf.py

@@ -72,7 +72,9 @@
 # This is processed by Jinja2 and inserted before each notebook
 nbsphinx_prolog = r"""
 {% set docname = 'docs/' + env.doc2path(env.docname, base=None) %}
-{% set git_ref = 'master' if '.' not in env.config.current_version else 'v' + env.config.release %}
+{% set git_ref = 'master' if not READTHEDOCS else 
+                 commit if '.' not in current_version else
+                 'v' + env.config.release %}
 .. raw:: html
 
     <div class="admonition note">