Allow servant-0.20 and other newer deps

[ysangkok]

Tested using

$ for i in servant-swagger-ui*; do (cd $i; cabal outdated); done
All dependencies are up to date.
All dependencies are up to date.
All dependencies are up to date.
All dependencies are up to date.
All dependencies are up to date.
$ cabal build --enable-tests --allow-newer=servant-blaze:servant -c 'base-compat>=0.13' all

There are no tests to run.

Depends on:

• https://github.com/haskell-servant/servant-blaze/pull/23