Add from: undefined to hide warnings from PostCSS

[pateketrueke]

I got this:

Without `from` option PostCSS could generate wrong source map and will not find Browserslist config. Set it to CSS file path or to `undefined` to prevent this warning.

This can be safely omitted with:

- .process(contents, { parser: safeParser })
+ .process(contents, { parser: safeParser, from: undefined })

Source: https://github.com/SparkPost/heml/blob/master/packages/heml-styles/src/index.js#L90

[lucasgranberg]

Also mentioned here: #62. I can confirm the error goes away with this. Ill create a PR.