Removes testing on R v3, and adds testing R devel

[hlapp]

Fixes #283.

[hlapp]

The question is whether we should also explicitly raise the R version requirement in DESCRIPTION to 4.x. In theory this would sounds like a trivial change, but I think this should nonetheless precipitate incrementing the version to 0.4.x. Otherwise, if incremented only to 0.3.1, it would signal that an automatic upgrade from 0.3.0 is OK when in reality it would break things in an R 3.x environment. Or perhaps it simply wouldn't get upgraded in that environment?

@johnbradley any thoughts either way?

[johnbradley]

I would raise the R version to 4.0. If our R3 tests failed to install dependencies something similar might happen to a user with R 3 installed. I would expect existing R3 installations to not install the new version of rphenoscape once we raise the R requirement to 4. Going with 0.4.x for our version to convey the bigger jump also sounds good since we aren't at 1.0.x yet.

[hlapp]

OK. I upgraded the required R version. I'll increment the version later once we perhaps fold a few more clean-ups into the upgrade (such as #285, and the checks also indicate some URLs that need fixing).

[johnbradley]

LGTM