Fix scp issues with non-POSIX pathnames on Windows

On non-POSIX operating systems such as MS Windows, the use of the
external program 'scp' (through child_process) creates uncertainty
on the pathname style that is expected by 'scp'. For instance,
references to drive letters vary between different implementations
(e.g. Cygwin vs GnuWin32 based).

The use of 'scp2' NPM package workarounds the issue by alleviating
the need for the external program.