exclude postcss-cssnext from default config

[SPAHI4]

I got an error when try to provide the new instance of postcss-cssnext with my custom config.

Also, the last version of postcss-nesting doesn't support for postcss 5.x.

[giuseppeg]

Can you paste the error on gist.github.com and share the link here?

Also, the last version of postcss-nesting doesn't support for postcss 5.x.

See my reply /issues/18