Add an RBinaryInstall command, and use it with c2d4u.

This patch does three main things:
 * add c2d4u to our apt configuration,
 * add a command to install a binary version of an R package from c2d4u, and
 * use c2d4u for devtools.

I also cleaned up one or two printed messages.
Convert r_binary_install arg to lowercase.
Add some fixes for OSX.
Update the test after updating the source. Classy.
Add multiple args for r_binary_install.
bash. yep.
bash. yep. again.
bash. yep. again. yet again.
Make a message more readable.
Merge pull request #48 from romainfrancois/master

dump results of tests
Variable rename.
Merge pull request #47 from craigcitro/r_binary_deb

Add an RBinaryInstall command, and use it with c2d4u.
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.