fix(wcontrol_check_pgversion): Use version_compare() for Pg's version check (refs #5018)

Use PHP's version_compare() to perform Pg's version checks instead of
using custom comparison which does not handle versions like "9.4beta2".
Merge pull request #110 from eguaj/1.5-A5018

fix(wcontrol_check_pgversion): Use version_compare() for Pg's version check (closes #5018)