(maint) Don't cd into pkg/pe/deb unless it exists

Previously the pe_ship task tried to cd into pkg/pe/deb even if it didn't
exist. This would cause failures for projects like pe-razor-server, which would
never have any debs to ship. This commit updates the logic so that no directory
change happens unless the target directory exists.
Merge pull request #244 from haus/fix_pe_ship_debs

(maint) Don't cd into pkg/pe/deb unless it exists