Only cat test output if it exists.

Fixes #49.
Fix missing quotes.
Don't de-dupe an expression. Quoting is hard.
Make the `R CMD BUILD` and `R CMD check` args configurable as env vars.
Temporarily switch CRAN mirror.
fiddling
testing
test env vars in .travis.yml.
another test
empty test
code review, back to rstudio cran mirror
Merge pull request #50 from craigcitro/test_config

Make build and check arguments configurable via an environment variable.
Merge branch 'master' into osx
Forgot a quote in a comment.

[ci skip]
Merge branch 'master' into osx
Go back to default args on OSX.

We don't do anything with tex on OSX, so this is safest.