Version bumps: 5.2 -> 5.4 5.3 -> 5.5

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

	gnuplot stable version
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Version 5.2.8 went out earlier this month.

Initial release of 5.4 is projected for Spring 2020.
I have created branch-5-4-stable for staging of release candidates.

	gnuplot development
	____________________

Development continues in the main branch ("master") in the git repository.
The identifying version for the development branch has been bumped
from 5.3 to 5.5 so that it stays ahead of the stable branch.

	Ethan