servant-fragment 0.15.3 requires servant >= 0.18.2

The `Fragment` type is not exposed in older of `servant`. E.g. with `servant-0.18` I get the following compiler error:
```
Configuring library for servant-foreign-0.15.3..
Preprocessing library for servant-foreign-0.15.3..
Building library for servant-foreign-0.15.3..
[1 of 3] Compiling Servant.Foreign.Internal ( src/Servant/Foreign/Internal.hs, dist/build/Servant/Foreign/Internal.o )

src/Servant/Foreign/Internal.hs:335:29: error:
Not in scope: type constructor or class ‘Fragment’
|
335 | => HasForeign lang ftype (Fragment a :> api) where
| ^^^^^^^^

src/Servant/Foreign/Internal.hs:336:23: error:
Not in scope: type constructor or class ‘Fragment’
|
336 | type Foreign ftype (Fragment a :> api) = Foreign ftype api
| ^^^^^^^^
```