Problem
-------

`autoprefixer-rails` v 9.x was reporting deprecation warnings everytime
you did a build or serve.

```
DEPRECATION WARNING: autoprefixer-rails was deprected. Migration guide:
https://github.com/ai/autoprefixer-rails/wiki/Deprecated (called from load at /Users/jon/.rbenv/versions/2.6.5/bin/middleman:23)
```

Some futher investigation makes me think it's not in fact deprecated but
has picked up some new owners and is still in development.

Solution
---------

Bump to the new version (10.0.1) to suppress the error message.

I tested this locally by building a current middleman project and the
css that came out was identical to the previous version.  Also your
tests still pass.

This reverts commit f9819a6.