Get builds to run on all supported R versions #1657

Job	Run time
R-CMD-check / macOS-latest (release)	4m 16s
R-CMD-check / windows-latest (devel)	8m 47s
R-CMD-check / windows-latest (release)	8m 47s
R-CMD-check / windows-latest (oldrel)	6m 43s
R-CMD-check / ubuntu-latest (devel)	10m 14s
R-CMD-check / ubuntu-latest (release)	6m 20s
R-CMD-check / ubuntu-latest (oldrel)	4m 45s
R-CMD-check / ubuntu-latest (oldrel-2)	5m 48s
R-CMD-check / ubuntu-latest (oldrel-3)	8m 49s
R-CMD-check / ubuntu-latest (3.6)	4m 37s
	1h 9m 6s

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