Replace deprecated sourceCmd with sourceCmdWithConsumer

The [`process-conduit`](http://hackage.haskell.org/package/process-conduit-1.2.0.1) package was recently deprecated, so I replaced the usage of [`sourceCmd`](http://hackage.haskell.org/package/process-conduit-1.2.0.0/docs/Data-Conduit-ProcessOld.html#v:sourceCmd) with [`sourceCmdWithConsumer`](http://hackage.haskell.org/package/conduit-extra-1.1.2/docs/Data-Conduit-Process.html#v:sourceCmdWithConsumer) from the latest version of [`conduit-extra`](http://hackage.haskell.org/package/conduit-extra-1.1.2).