fix subscriptionId not sent when calling dispose() on relay-socket

[davidye]

This should fix #13

[tomconroy]

@mgtitimoli @benwilson512 is someone on the absinthe team available to review these open PRs? This javascript seems essential for working with absinthe subscriptions

[mgtitimoli]

Thanks for having taken the time to contribute.

Closing this as it does not address the real issue.

You can find the next release at #33.