Update dependencies

[ai]

@robwierzbowski thanks for your amazing work for this plugin.

I updated dependencies. The most important update is Browserslist. Old Browserslist is slow (@kgrz found performance impact from node-pixrem) and could throw an error on new queries (like very useful `not dead).

PostCSS and reduce-css-calc are less important but it will be good to update them.

What version should be used to release it:

1. Browserslist 2.0 → 4.0 will not change public API for this plugin. So you do not need a major bump for it.
2. PostCSS 6.0 → 7.0 does not require any plugin changes. PostCSS 7.0’s plugin will be still compatible with PostCSS 6.0. So you do not need a major bump for it.
3. reduce-css-calc 1.0 → 2.0 changes. I didn’t find critical changes.

As result, I think minor changes will be OK (especially, because it will fix Browserslist error on new queries).

[ai]

@robwierzbowski seems like we need a major release since PostCSS 7.0 does not support Node.js 4

[robwierzbowski]

Updates all look good to me. I'll merge and release a major.