Fix from source quickstart instructions for Windows
See:
+qs({package: 'source'}) git clone https://github.com/explosion/spaCy
+qs({package: 'source'}) cd spaCy
- +qs({package: 'source'}) export PYTHONPATH=`pwd`
+ +qs({package: 'source', os: 'mac'}) export PYTHONPATH=`pwd`
+ +qs({package: 'source', os: 'linux'}) export PYTHONPATH=`pwd`
+ +qs({package: 'source', os: 'windows'}) set PYTHONPATH=/path/to/spaCy
+qs({package: 'source'}) pip install -r requirements.txt
+qs({package: 'source'}) python setup.py build_ext --inplace
