Removes the source PostCSS warnings on discarding the source maps

[phoulgaux]

The problem that is solved here is the following:

1. When options.sourceMap == false is set for postcss-loader, there is a warning about discarding source maps from sass-loader.
2. CircleCI has CI=true, which makes build scripts treat warnings as errors and those PostCSS warnings break the build.
3. The current config discards the source maps in any case so I disabled generating them in the first place.