Remove opal-browser dependency

[wied03]

In order to keep the core GEM light, might want to see if this can be removed and if necessary, put it on a higher level GEM.

[ajjahn]

@wied03 I agree with this. Opal browser is only gaining the ability to attach components to browser Elements. Really, we could continue to support that, but remove the requirement. Users can add depend on opal-browser specifically if they want that behavior.

[sollycatprint]

This issue was moved to ruby-hyperloop/hyper-react#133