Error: TypeError: decl.moveTo is not a function

I'm looking into using cssnano via `rollup-plugin-postcss`, with the `minimize: true` option, but I'm encountering an error whenever I try to use it:

```
[!] (postcss plugin) TypeError: decl.moveTo is not a function
client_src/css/main.css
TypeError: decl.moveTo is not a function
at path/to/project/node_modules/leaflet/dist/leaflet.css:188:10
at path/to/project/node_modules/cssnano/node_modules/postcss-merge-rules/dist/index.js:246:22
at path/to/project/node_modules/postcss-import/node_modules/postcss/lib/container.js:135:18
at Rule.each (path/to/project/node_modules/postcss-import/node_modules/postcss/lib/container.js:101:16)
at Rule.walk (path/to/project/node_modules/postcss-import/node_modules/postcss/lib/container.js:131:17)
at path/to/project/node_modules/cssnano/node_modules/postcss-merge-rules/dist/index.js:242:18
at path/to/project/node_modules/postcss/lib/container.js:237:28
at path/to/project/node_modules/postcss/lib/container.js:144:26
at Root.each (path/to/project/node_modules/postcss/lib/container.js:110:22)
at Root.walk (path/to/project/node_modules/postcss/lib/container.js:143:21)
at Root.walkRules (path/to/project/node_modules/postcss/lib/container.js:235:25)
```

After a bunch of digging, it transpired that the issue was with `postcss-merge-rules`, which was reported and fixed in ben-eb/postcss-merge-rules#52. Confused, I turned to `npm-why` to figure out what was going wrong. It turns out that `cssnano` is still depending on version 2.1.2 of `postcss-merge-rules` - even though the latest version is 4.0.2!

```
$ npm-why postcss-merge-rules

Who required postcss-merge-rules:

my-project > rollup-plugin-postcss > cssnano > postcss-merge-rules@2.1.2
```

However, there's an additional twist in the tale. Upon reporting this to the `cssnano` project in cssnano/cssnano#676, I discovered that the issue wasn't with cssnano at all - it's with this plugin (rollup-plugin-postcss) - which is depending on an ancient version of cssnano.

Could cssnano be updated please? I'd like very much to be able to minimise my css, but I can't do so currently :-/

More information is, of course, available upon request.