Bump http version to 4.0

[radar]

Given that this library only uses the HTTP.get method, I think it's safe to bump this version. The tests all seem to pass.

[fanfilmu]

Just for future reference @radar , this gem may use only get and update is safe, but since you specified this a dependency, you require everybody who uses creek to update HTTP gem to 4.0.0 as well, and this might not be that safe and easy ;)