Move swagger2 helper function upstream.

[fisx]

This will work better after GetShopTV/swagger2#195 has been merged, which will also require some minor changes here. Just opening this to keep track of what's going on upstream.

[fisx]

@arianvp could you take a quick look? the actual change is trivial, i'm just wondering if the version bounds driven by the change are acceptable, or whether we want to be more backwards compatible.

[akhesaCaro]

Hi,
Servant-swagger will be moved into the main Servant repo (see : haskell-servant/servant#1475)
If this issue is still relevant, would it be possible for you to rebase on that repo (when it will be merged)? : https://github.com/haskell-servant/servant/

Thanks in advance!