Require version 1 of parskip.sty

Newer versions change the spacing after headings which causes
different page breaks.

Backported from master, commit 4ebd8cb.

--- a/pms.cls
+++ b/pms.cls
@@ -17,6 +17,7 @@
 % Enable UTF-8 input encoding
 \PassOptionsToPackage{utf8}{inputenc}
 % algorithmic and algorithm to be loaded last to avoid failures
+\RequirePackage{parskip}[=v1] % Space between paragraphs instead of indentation
 \RequirePackage{%
     fontenc,	% Load extended font sets
     isodate,	% Date formatting and conversions
@@ -30,7 +31,6 @@
     lscape,	% Rotating pages
     marginnote, % Typeset a paragraph in the page margin
     paralist,	% Additional list environments
-    parskip,	% Space between paragraphs instead of indentation
     verbatim	% Extend the print-as-is functionality
 }