Use global scope when referring to classes instead of local scope, avoiding conflicts with other gems

[bogdanRada]

Hello,

Thank you for this great gem. I was trying to use this gem with sprockets-helpers gem

And i found one issue , after requiring `sinatra/sprockets/helpers`` from that gem,
i received this error:

gems/sinatra-asset-pipeline-1.0.0/lib/sinatra/asset_pipeline.rb:7:in `registered': uninitialized constant Sinatra::Sprockets::Environment (NameError)

This fixes this issue. Let me know what you think. Thank you very much.

[kalasjocke]

@bogdanRada Do I understand this pull request correctly if I say that the sprockets-helpers gem define it's own Sprockets module which causes Sinatra::Sprockets::Environment to be undefined? If so, let's merge this one.

[bogdanRada]

Yes, that is true :)

[kalasjocke]

@bogdanRada Great! Thanks for the pull request, merging.

[bogdanRada]

Thank You very much