Update autoprefixer-core to the latest version to remove console warnings

[piotrkulpinski]

Hi there,
I'm getting following warnings while using cssnext:

Container#eachAtRule is deprecated. Use Container#walkAtRules instead.
Container#eachDecl is deprecated. Use Container#walkDecls instead.
Container#eachRule is deprecated. Use Container#walkRules instead.
Node#removeSelf is deprecated. Use Node#remove.
Container#eachDecl is deprecated. Use Container#walkDecls instead.
Node#style() is deprecated. Use Node#raw()
Node#before is deprecated. Use Node#raws.before

I found out that it's related to autoprefixer itself and it was fixed in v6 of the package, but cssnext still uses the old v5 version.

Here's the reference: postcss/autoprefixer#500

[ben-eb]

Duplicate of #202.

[piotrkulpinski]

That one is closed already and I don't see my issue resolved there so I think it should stay for now.

[ben-eb]

See also #208.

[MoOx]

Please see #230