fix: Fix broken UMD build

[pimterry]

npm run build is currently broken, for two reasons:

• browser was removed from package.json recently (d3b30fd), but it's used in rollup.config.js

• minify && uglify becomes undefined for the non-minified build, which makes rollup-plugin-commonjs throw this (at https://github.com/rollup/rollup-plugin-commonjs/blob/v9.3.4/src/resolve-id.js#L50), because undefined isn't a valid plugin:

  [!] TypeError: Cannot read property 'resolveId' of undefined
  TypeError: Cannot read property 'resolveId' of undefined
      at .../react-sortable-hoc/node_modules/rollup-plugin-commonjs/src/resolve-id.js:52:6
  

  (No idea why this wasn't always happening in the past 🤷‍♂️)

This PR is a quick fix for both. umd:main has the same value browser used to have, so that's easy, and replacing undefined with {} makes plugin.resolveId undefined, which means the plugin gets dropped by rollup, which is really what we want anyway. With this, the build now passes on my machine with a fresh install.

[clauderic]

Hey @pimterry, thanks for this PR ❤️