Get builds to run on all supported R versions #1658

Job	Run time
R-CMD-check / macOS-latest (release)	4m 29s
R-CMD-check / windows-latest (devel)	9m 41s
R-CMD-check / windows-latest (release)	9m 13s
R-CMD-check / windows-latest (oldrel)	9m 36s
R-CMD-check / ubuntu-latest (devel)	11m 7s
R-CMD-check / ubuntu-latest (release)	6m 47s
R-CMD-check / ubuntu-latest (oldrel)	8m 49s
R-CMD-check / ubuntu-latest (oldrel-2)	5m 47s
R-CMD-check / ubuntu-latest (oldrel-3)	9m 47s
R-CMD-check / ubuntu-latest (3.6)	7m 0s
	1h 22m 16s

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