Change isomorphic-unfetch to track latest version again

[pheuberger]

In PR #104 we pinned isomorphic-unfetch to version 4.0.0 because of a bug this PR in the unfetch library is solving which isomorphic-unfetch depends on.

We should keep an eye on the PR and roll-back the downgrade introduced in 6c8c6a0 once the PR is merged.