build on the previous base image
build on the previous base image

as requested in
https://github.com/appveyor/ci/issues/268#issuecomment-101665264

stephenrauch/pydal,niphlod/pydal,willimoa/pydal,manuelep/pydal,kmcheung12/pydal,web2py/pydal,michele-comitini/pydal