Travis doc build not run with master anymore
Since a few commits (but I don't think it is related to one of our commits, rather to conda or so), the docs are run with pandas 0.19.2, and not with master (giving a lot of errors for those code blocks that show new things, see eg [here](http://pandas-docs.github.io/pandas-docs-travis/whatsnew.html#dtype-keyword-for-data-io))

The cause seems to be that the installing of `r` and `rpy2` seems to install pandas (https://travis-ci.org/pandas-dev/pandas/jobs/198582770, line 1233 in the log). Before that commit, the build log does not include pandas in that conda output. 
The strange thing is also that pandas appears in the conda output as a 'new' package, not one that is up or downgraded. Which seems to indicate that pandas was not yet installed, although the master version seemed to be installed correctly.