Update webpack version in peerDependencies

[RomanHotsiy]

When using the plugin with webpack >=2 npm warns about unresolved peer dependencies:

npm WARN string-replace-webpack-plugin@0.0.4 requires a peer of webpack@^1.4.2 but none was installed.

I've added >=2.2.0-rc.0 to the range. I've chosen 2.2.0-rc.0 because it seems that there were some issues with 2.1.0-beta.27.

[jamesandersen]

Thanks @RomanGotsiy! just merged and published to npm

[RomanHotsiy]

@jamesandersen that was fast 🚅 ! Thanks