pkg/debian: Fix versioning if upstream is dev/rc

  * Ensure a clean upgrade path on debian:
   * Change the upstream version from 'x.x.x.dev' and 'x.x.x.rcx' to 'x.x.x~dev' and 'x.x.x~rcx'
   * Use UTC instead of local time in the debian version
Merge pull request #662 from pieterlexis/debian_upstream_dev_version

pkg/debian: Fix versioning if upstream is dev/rc