Gnuplot 5.4.8 release

[Ethan A M. <me...@uw...>]

Gnuplot 5.4.8

I have bumped the reported version to 5.4.8 and uploaded a new tarball to
SourceForge.

This repairs an inconsistancy in the source and binary packages for 5.4.7,
in which

Windows binaries built from the source package identified as version
"5.4.7alpha"

rather than  "5.4.7".  There is no change to the gnuplot source between
5.4.7 and 5.4.8.

    https://sourceforge.net/projects/gnuplot/files/gnuplot/5.4.8

I apologize for introducing this inconsistancy when preparing the 5.4.7
relaase package.

    Ethan