enable pip cache in appveyor build
enable pip cache in appveyor build

hugovk/coveragepy,blueyed/coveragepy,hugovk/coveragepy,nedbat/coveragepy,blueyed/coveragepy,hugovk/coveragepy,hugovk/coveragepy,nedbat/coveragepy,nedbat/coveragepy,nedbat/coveragepy,hugovk/coveragepy,blueyed/coveragepy,blueyed/coveragepy,nedbat/coveragepy,blueyed/coveragepy