Recommend local installation

[Rich-Harris]

Via rollup/rollup#1082. Installing locally increases the cost of using a filewatching abstraction like chokidar or pathwatcher, because they can be a bit of a pain sometimes due to their dependency on native modules, particularly in a CI context. But it does mean we can get rid of the version check.

If we do find we need to go down the chokidar/whatever route, one option would be to have a rollup-watch-chokidar package, and rollup --watch chokidar could tell the CLI which watcher to use.