DOC: update sphinx requirements for doc building
Simplified the version requirement for sphinx (>= 1.3.2). In principle >= 1.2.2 but < 1.3.0 also works, but left it out for simplicity.

@cswarth Do you have other things that you find unclear about the requirements for the doc build environment?

I don't think clarity is the problem as much as being a relatively high barrier to entry for contributing to the docs.  By the time you get done installing all the prerequisites and optional dependencies, it feels quite onerous.  I used pip to install the optional dependencies as I ran across them, but often wished there were a requirements.txt (or more likely, Dockerfile) to establish a known working environment.  
 We could change the suggestions of `conda create -n pandas_dev python=3 --file ci/requirements_dev.txt` to `ci/requirements_all.txt`
 docker is way too heavyweight for anything like this
 > We could change the suggestions of conda create -n pandas_dev python=3 --file ci/requirements_dev.txt to ci/requirements_all.txt

That is already mentioned in the section about creating a development environment (http://pandas-docs.github.io/pandas-docs-travis/contributing.html#creating-a-development-environment), but it would maybe made more prominent in the section on doc building requirements (instead of just linking to that section)
 I only mentioned a dockerfile because I found there were extra-pythonic dependencies that were not captured by pip or conda.   For instance, I couldn't install matplotlib unless I first ran `apt-get install pkg-config` and `apt-get install libfreetype6-dev`. 
 not true conda installs everything u need. if you are using pip then YOU do need to manage dependencies yourself. and that's why we don't use pip for anything.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13532?src=pr) is **84.33%**

> Merging [#13532](https://codecov.io/gh/pydata/pandas/pull/13532?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

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> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [8f8d75d...d30cc3f](https://codecov.io/gh/pydata/pandas/compare/8f8d75d315f5a61bbf2060ff821bff9b282a1582...d30cc3f402535c3393caa2937c27f4fcb05b5594)
 > I only mentioned a dockerfile because I found there were extra-pythonic dependencies that were not captured by pip or conda.

@cswarth If you had problems installing matplotlib with conda, you can report it over there, but normally this should just work.

I personally think the instructions of how to set up a development environment are already quite elaborate, but improvements are always welcome! 

I reworded the reference to first creating a dev environment for building the docs to make this more clear.
