Do we always need the assertthat package?

If not, it should not be part of the minimal sample.
Merge pull request #67 from krlmlr/assertthat

Do we always need the assertthat package?
treat warnings as errors
add environment option that enables treating warnings as errors, off by default

per Dirk's request
need to set OS also in .travis.yml for tests
Merge pull request #72 from krlmlr/test-glitch

need to set OS also in .travis.yml for tests
precisely specify log file to check
Merge pull request #71 from krlmlr/69-warnings-are-errors

treat warnings as errors
added dropbox script
r-binary-install uses r-install as fallback if not running Linux or FORCE_SOURCE_INSTALL is set
can't use -o when testing with [[
Merge pull request #79 from romainfrancois/master

added dropbox script
Merge pull request #78 from krlmlr/source-install

r-binary-install uses r-install as fallback if not running Linux or FORCE_SOURCE_INSTALL is set
retrying apt-get update with exponentially increasing time intervals until success
maximum time interval between retries is one minute
Retry all apt-get calls, too
Merge pull request #77 from krlmlr/retries

retry fetching files with exponentially increasing timeouts
Tweak ordering. Add a few more useful examples.
Merge pull request #82 from hadley/patch-3

Tweak ordering. Add a few more useful examples.
Merge remote-tracking branch 'parentrepo/master'