appveyor.yml: Run pip install as Python module
appveyor.yml: Run pip install as Python module

pip now complains about being downgraded when being
executed via the `pip` command on Windows.

Closes https://github.com/coala/coala/issues/5466

coala-analyzer/coala,jayvdb/coala,SanketDG/coala,SanketDG/coala,coala-analyzer/coala,coala/coala,SanketDG/coala,jayvdb/coala,coala/coala,coala/coala,jayvdb/coala,coala-analyzer/coala