Error building in mybinder

## Bug report
#### Description

Demo container not building in binder

```
Removing intermediate container f8f6e2fb55fc
Step 25/27 : USER ${NB_USER}
---> Running in cf2a7e8109e6
---> 6a0841b27202
Removing intermediate container cf2a7e8109e6
Step 26/27 : RUN conda env update -n root -f "environment.yml" && conda clean -tipsy
---> Running in 57ad6dd7cfda
Fetching package metadata .............
Solving package specifications: .

UnsatisfiableError: The following specifications were found to be in conflict:
- openbabel >=2.3,<2000 -> python 2.7*
- python 3.6*
Use "conda info " to see the dependencies for each package.

```
#### Steps to reproduce

https://mybinder.org/v2/gh/Autodesk/molecular-design-toolkit/master

Start by reproducing the Binder build from the linked repository URL and inspect environment.yml, which is used in the failing conda update step. Check the reported conflict between openbabel and Python 3.6; done means the demo container builds successfully in Binder.

Written by the indexing model from the issue text.