CI: doc build on travis is failing on installation (r channel)
Since yesterday or so (eg https://travis-ci.org/pandas-dev/pandas/jobs/430680571), the doc build fails on installation:

```
[create env]
Solving environment: ...working... failed
CondaHTTPError: HTTP 404 NOT FOUND for url <https://conda.anaconda.org/r/noarch/repodata.json>
Elapsed: 00:00.034960
CF-RAY: 45ce57b9b8c12180-EWR
The remote server could not find the noarch directory for the
requested channel with url: https://conda.anaconda.org/r
As of conda 4.3, a valid channel must contain a `noarch/repodata.json` and
associated `noarch/repodata.json.bz2` file, even if `noarch/repodata.json` is
empty. please request that the channel administrator create
`noarch/repodata.json` and associated `noarch/repodata.json.bz2` files.
$ mkdir noarch
$ echo '{}' > noarch/repodata.json
$ bzip2 -k noarch/repodata.json
You will need to adjust your conda configuration to proceed.
Use `conda config --show channels` to view your configuration's current state.
Further configuration help can be found at <https://conda.io/docs/config.html>.
```