Add linting on R code

[jameslamb]

We should use a linter to somewhat-standardize the style of the R code in this project. For R code, I'm fond of the covr package.

To close this issue, introduce linting in the tests run on Travis. This issue will involve reading our .travis.yml and the scripts in .ci/ to get an understanding of how uptasticsearch's Continuous Integration (CI) setup.

I recommend taking an approach similar to the one in microsoft/LightGBM#2437, but it's up to you (the contributor) to propose how you want to handle it.

This is a medium-difficulty issue, but one you can do without understanding anything about Elasticsearch or uptasticsearch's R code.