DOC: doc build on travis timing out
For some reason, the doc build on travis started timing out since a couple of commits. See eg https://travis-ci.org/pandas-dev/pandas/builds/206994038. It just takes so long it hits the "The job exceeded the maximum time limit for jobs, and has been terminated." (around 50 min), while before the doc build on travis took between 20 and 30 min.

It started with the FrozenList changes (https://github.com/pandas-dev/pandas/commit/37fe2c4edddbec2c08d561667897a1ef5a18771c), but I don't see how that could cause this. 

I also don't directly see any changes in the environment that is created on travis to build the docs (eg both using sphinx 1.5.1, no change there).