I'd prefer to do the doc check prior to the package check so that we can
fail fast in light of trivial errors, but I worry about the side-effects
of installing devtools and roxygen2 on our tests, so I'm punting those
tasks until after our CMD check.
It may be possible to parallelize this work adjacent to our package
check (and only do it on one version of R rather than all three), but I
haven't explored that yet.
Failures in the `after_script`s don't fail the build, surprisingly. The
`|| travis_terminate 1` accomplishes that. `travis_terminate` taken
from
https://github.com/travis-ci/travis-ci/issues/1574#issuecomment-164094347
`git clean` incantations found in
https://issues.jenkins-ci.org/browse/JENKINS-31924
