Bump version to 18.1
Bump version to 18.1

pfmoore/pip,pradyunsg/pip,pradyunsg/pip,rouge8/pip,sbidoul/pip,pfmoore/pip,xavfernandez/pip,rouge8/pip,xavfernandez/pip,sbidoul/pip,pypa/pip,xavfernandez/pip,pypa/pip,rouge8/pip