Exposing the version argument of saveRDS in expect_known_value()

[lorenzwalthert]

This could be helpful because R < 3.6 can't read version 3, and for that reason, newly created reference objects (or updated ones) in a developer environment with R >= 3.6 have to be manually read and written back in version 2 for tests to pass on R < 3.6.

[jennybc]

Also affects expect_equal_to_reference()

I now realize that expect_equal_to_reference() is basically soft-deprecated in favour of expect_known_value().