Replace https://github.com/satori/go.uuid

[sgotti]

Should use maintained fork from https://github.com/gofrs/uuid

Since this new version returns an error when calling uuid.NewV4 we should choose if we want to change the code to handle the error or just panic (since it'll mean that uuid failed to read from system random number generator) by simply replacing uuid.NewV4() with uuid.Must(uuid.NewV4())

[sgotti]

Fixed by #311