Fix build by removing flake8-rst
Seems like `flake8-rst==0.4.2` is not available in conda anymore. This is making the Travis build fail. Newer versions of `flake8-rst` are not usable yet, as they validate more things than `0.4.2` and validating our docs would fail the CI too.

We're working on fixing all the flake8-rst linting problems in the docs, and soon we'll be able to upgrade to a newer version. But for now, I think removing `flake8-rst` from the dependencies, and disabling the docs validation in the CI is the best, so Travis builds pass again.

CC: @jreback @FHaase 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23975?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@30c1290`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23975/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23975?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23975   +/-   ##
=========================================
  Coverage          ?   92.31%           
=========================================
  Files             ?      161           
  Lines             ?    51473           
  Branches          ?        0           
=========================================
  Hits              ?    47517           
  Misses            ?     3956           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (?)` | |
| #single | `42.43% <ø> (?)` | |


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23975?src=pr&el=footer). Last update [30c1290...4402cb1](https://codecov.io/gh/pandas-dev/pandas/pull/23975?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think we could upgrade flake8-rst and exclude all files that throw errors.

Then we could reactivate it file by file. So if #23847 could be merged I'd rather merge that. Why isn't it available anymore? Was it previously coming through the conda-forge channel? Yes. It still shows 0.4.2 in the [files](https://anaconda.org/conda-forge/flake8-rst/files) section. This shouldn't be necessary anymore. Anaconda is in the process of switching how conda-forge packages are being mirrored.

Follow https://gitter.im/conda-forge/conda-forge.github.io for updates. latest build seems ok. migth have been temporary? Yeah. It sounds like the repodata.json and the actual package location got
out of sync when they flipped the mirroring. It's been reverted for now.

On Wed, Nov 28, 2018 at 4:23 PM Jeff Reback <notifications@github.com>
wrote:

> migth have been temporary?
>
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 I'm still having some problems with travis, it's timing out when creating the environment that has all the linting stuff. But closing this.