Use an older version of wheel for Python 3.5

Seems the current version of `wheel` does not exist as a `conda` package
in either `defaults` or `conda-forge`. So downgrade a little bit to get
a version of `wheel` that works across Python versions.

environment_ci.yml

@@ -5,7 +5,7 @@ channels:
 
 dependencies:
   - pip==18.1
-  - wheel==0.32.2
+  - wheel==0.32.0
   - coverage==4.5.2
   - pytest==3.10.1
   - cython==0.29