Upgrade Immutable to the latest version

[andrei-cacio]

As of version 3.7.6 there are new APIs introduced, like the insert() method on Listss.

[raulmatei]

This should be fixed by #214, which removes the ImmutableJS dependency and adds it as a peer dependency. It will be then the developer's choice on which version to install (the minimum version is 3.7.3, which is the current version used by NuclearJS)

[andrei-cacio]

Thanks @raulmatei. #214 is a better fix for this problem