Run tests twice, and make warnings errors

- First test runs all skip_on_cran() test
- Second test runs exactly as cran, and skips them.
- Warnings are now errors and cause a travis failure.  I wonder if I
can makes NOTES errors as well?

Based on:
https://github.com/topepo/caret/pull/92
https://github.com/rOpenGov/pxweb/blob/master/.travis.yml
Merge pull request #122 from zachmayer/travis-update

Run tests twice, and make warnings errors