Bump R version to 3.5

[jgabry]

In R cmd check we now get this:

  NB: this package now depends on R (>= 3.5.0)
  WARNING: Added dependency on R >= 3.5.0 because serialized objects in
  serialize/load version 3 cannot be read in older versions of R.
  File(s) containing such objects:
    ‘loo/tests/testthat/data-for-tests/kfold-calibrated.Rds’
    ‘loo/tests/testthat/data-for-tests/kfold-miscalibrated.Rds’

We could replace these files using saveRDS(..., version = 2), which should get rid of this warning, but I think it's also fine to just require R 3.5.0 since that's still a very old version of R. In fact there may even be other things in the package at this point that require later versions of R that we're not aware of.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.70%. Comparing base (05a6bd2) to head (8e3e95e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #364   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files          31       31           
  Lines        3029     3029           
=======================================
  Hits         2808     2808           
  Misses        221      221           

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8e3e95e is merged into master:

• ✔️loo_function: 1.48s -> 1.48s [-0.94%, +0.99%]
• 🚀loo_matrix: 1.1s -> 1.09s [-1.27%, -0.06%]
  Further explanation regarding interpretation and methodology can be found in the documentation.