Issue title: _autoprefixerPrefix error with node v7
 Issue body:  Cannot read property '_autoprefixerPrefix' of undefined
This error occurs with Node v7.10.0. With node v6 everything works fine



 Comments: 
 Comment 0: Version 2 now supports PostCSS 6. Would you try again?
 Comment 1: Seems it is still not fixed for me.
I updated package.json, deleted npm, reinstalled modules and error still exists.
(There was deprecation warning about Node.raws it was fixed after update)

I use Node v7 if it matters
 Comment 2: Thanks for the update. I’m wondering if it’s another plugin in your stack. What other plugins are you using? I only ask because this project does not use Autoprefixer, and your error indicates something with Autoprefixer.
 Comment 3: I'm using only postcss-normalize and autoprefixer.
I made a little test. This is strange, but everything works fine, if normalize goes after autoprefixer.
But if normalize goes first, error occurs.
Like this:
```
.pipe(postcss([
    postcssNormalize(),
    autoprefixer()
]))
```
 Comment 4: Testing this now.
 Comment 5: @jonathantneal what I should test? `master`?
 Comment 6: This should be resoled in 2.0.1.

Test: https://github.com/jonathantneal/postcss-tests/blob/master/postcss-normalize-test-00

Test Plugins: https://github.com/jonathantneal/postcss-tests/blob/master/postcss-normalize-test-00/test.js#L8-L9
 Comment 7: Thanks for fast fix, everything works now
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