Add nbsphinx to conda yaml

Update the conda_env yaml
Update environment

docs/environment.yml

@@ -1,9 +1,12 @@
 name: notebook_docs
 dependencies:
-- sphinx>=1.3
+- python=3.5
 - sphinx_rtd_theme
 - jinja2
 - tornado
 - nbformat
 - jupyter_client
 - ipykernel
+- pip:
+  - sphinx>=1.3.6
+  - nbsphinx

readthedocs.yml

@@ -1,4 +1,4 @@
 conda:
-    file: docs/conda_env.yml
+    file: docs/environment.yml
 python:
     version: 3