Update for the new Autoprefixer

[gisu]

The new autoprefixer requires PostCSS 7, so the dependencies have been updated. During the test node 4 was removed, the PostCSS 7 works with Spread Operator, which causes errors with node 4.

[coveralls]

Coverage remained the same at 100.0% when pulling 31de6e9 on gisu:master into 3a87a1c on postcss:master.

[w0rm]

@gisu thanks! will release a new version shortly, I guess it should be a major bump.