Use a lowercase 'immutable' module name

Webpack complains about `require('Immutable')`:

```
WARNING in ./~/immutable/dist/Immutable.js
There is another module with an equal name when case is ignored.
This can lead to unexpected behavior when compiling on a filesystem with other case-semantic.
Rename module if multiple modules are expected or use equal casing if one module is expected.
```

This pull request fixes the issue.