add r-htmltools pkg

[terhorst]

This is another conda package which is required for some tidyverse functions (specifically, stringr::str_view) but not included in the r-tidyverse metapackage. (cf #532)

[parente]

I think the build failure is related to the max build time allowed on Travis. I've opened #577 so we can address that problem separately.

I haven't had a lot of time recently to contribute to open source and don't look to have a lot coming up. That said, I don't want this PR to block waiting for the Travis fix. We can move ahead with merging it. If there is a true problem with the Docker image, it will simply fail to build on Docker Cloud and won't be available for people to use anyway.

If this sounds reasonable, let me know and I (or someone) can hit the merge button.