Test with R 3.1 ... 3.5

[krlmlr]

PR against master on purpose: You can maintain different versions of .travis.yml on both branches, that's the only way I know to have different build matrices on different branches.

When you merge this into development, you can revert this one commit c268cb2 in development to make sure that .travis.yml is different and stays different.

[brodieG]

@krlmlr the development branch now implements a fix for this that only runs the extensive tests on the rc branch, which in my workflow is the pre-deploy branch. Thanks again for your help here. This issue will auto-close when I push to master. Hoping to get that done and out to CRAN in the next few days. Sorry for the delays, I kept running into issues, etc.