Native R build on Travis

[cpsievert]

This will take advantage of "officially supported" native R builds on Travis

[cpsievert]

Since make now requires httr we'll need to install it on Travis. For this reason, I'd suggest merging this pull request before any others (otherwise the builds will likely fail).

[cpsievert]

Ah, I forgot Travis builds depend on devtools which depends on httr (as well as RCurl). I still think it's a good idea to list these in .travis.yml since that might change.

[cpsievert]

I'm ready to merge this when you are @tdhock @chriddyp

Only changes are to .travis.yml and .push_test_table.sh, so it won't be necessary to bump the package version

[tdhock]

+1

[tdhock]

this branch includes code for pushing log files to the plotly-test-table, which would have been helpful to see the plotly URLs in #193