Update tests.

A recent devtools update removed the support for bare repo names that
defaulted to `hadley/`; tests need updated to match.
Add support for installing pandoc in travis.

This adds code to optionally install `pandoc` in the travis worker, controlled
by the new environment variable `BOOTSTRAP_PANDOC`. The installed executable
is available in `${HOME}/opt/pandoc`, which is where `rmarkdown` will look for
it by default.

Works on mac and linux.
Cleanup create-osx.sh.
Merge remote-tracking branch 'craigcitro/master' into install-build