Fix ggplot2 dev

[cpsievert]

The development version of ggplot2 has shifted from proto objects to "ggproto" objects. That change breaks some assumptions we were making about the structure of ggplot objects.

[cpsievert]

Hey @chriddyp, does RTestBot3000 have a limit on the number of figures? I'm getting a bunch of server errors -> https://travis-ci.org/ropensci/plotly/builds/75853479

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/75859950

On TravisCI, commit b395eb4 was successfully merged with 1bd2e70 (master) to create b3ee71a. A visual testing table comparing 1bd2e70 with b3ee71a can be found here ->

http://cpsievert.github.io/plotly-test-table/R/b3ee71a/

Detected 0 differences

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/75860867

On TravisCI, commit b395eb4 was successfully merged with 1bd2e70 (master) to create a4d3d75. A visual testing table comparing 1bd2e70 with a4d3d75 can be found here ->

http://cpsievert.github.io/plotly-test-table/R/a4d3d75/

Detected 0 differences

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/75953419

On TravisCI, commit d1db211 was successfully merged with 1bd2e70 (master) to create 6ac8ae0. A visual testing table comparing 1bd2e70 with 6ac8ae0 can be found here ->

http://cpsievert.github.io/plotly-test-table/R/6ac8ae0/

Detected 0 differences

[cpsievert]

Abandoning this effort in favor of #269