fixup! Try fixing #231 again: Return to using platform_python_implementation

Author: gsnedders

.travis.yml

@@ -16,13 +16,9 @@ cache:
 env:
   - USE_OPTIONAL=true
   - USE_OPTIONAL=false
-  - USE_OPTIONAL=true SETUPTOOLS=18.5
   - USE_OPTIONAL=false SETUPTOOLS=18.5
-  - USE_OPTIONAL=true SETUPTOOLS=20.2.2
   - USE_OPTIONAL=false SETUPTOOLS=20.2.2
-  - USE_OPTIONAL=true SETUPTOOLS=20.6.6
   - USE_OPTIONAL=false SETUPTOOLS=20.6.6
-  - USE_OPTIONAL=true SETUPTOOLS=20.10.0
   - USE_OPTIONAL=false SETUPTOOLS=20.10.0
 
 before_install: