[scribus] maksvn script updated

[JLuc]

Following scribus 1.5 unstable official release, i updated maksvn script

https://github.com/JLuc/makscribus/blob/master/maksvn

'Maksvn 15' or 'maksvn 1.5' now compiles the official unstable 1.5.0 version
as scribus15.
Other options are identicals.
Without this new '15' option, the created exec defaults to 1.5.1svn head
and is now launched with scribus151svn (default name).

Both scribus15 and scribus151svn can run alongside each other.

This is tested on Ubuntu (and probably runs also on other linuxes ?)
JL