postcss-loader peer dependency should be more flexible
At present, postcss-loader peer dependency is locked down to version `2.1.3`. However, a patch version has been released.

I believe the peer dependency should be `^2.1.3`.

Also, is there a reason why all dependencies are locked to exact versions rather than being more flexible? There is a good chance that dependencies have changed at least in a minor or patch way, and the dependencies should be flexible enough for that, so that package managers can reuse dependencies accordingly.