Include previous R version in checks

A few weeks ago checks on the 3.5.3 rocker image stopped working. Enable checks on that version again.

added dev high priority tests labels

changed due date to April 30, 2020

mentioned in commit e87a90cb

mentioned in merge request !30 (merged)

changed the description

removed due date

changed milestone to %3.0

removed high priority label

closed via commit fdca7613

mentioned in commit 57633ff6

closed via merge request !30 (merged)