Add codecov on the R package

[jameslamb]

Should add some measure of code coverage on the R package (since that's our main published artifact) using codecov. This probably means adding codecov stuff to .travis.yml only for the one build with the most recent Elasticsearch version.

Should also add some rules to break PRs that don't test their code using a .codecov.yml file at the repo root.

Should also add a codecov badge in the README.

Fixing this issue doesn't have to be a single PR!