Upgrade swagger to 2.4.

Two changes:
- tags are now an insert-ordered hashset
- type_ .~ -> type_ ?~ with some magic type inference

This also updates stack.yaml to the newest GHC 8.6 LTS release.

stack.yaml

@@ -1,20 +1,4 @@
-# stack.yaml is used for circle-ci tests. Profiling build fails on circleci
-# with GHC 8.6, so we build with 8.4 for now.
-
-resolver: lts-13.29
-extra-deps:
-- Ranged-sets-0.4.0
-- configurator-pg-0.1.0.3
-- http-types-0.12.3
-- hasql-1.4
-- hasql-pool-0.5.1
-- hasql-transaction-0.7.2
-- text-builder-0.6.5.1
-- deferred-folds-0.9.10.1
-- primitive-0.6.4.0
-- jose-0.8.1.0
-- text-printer-0.5.0.1
-- network-2.7.0.1
+resolver: lts-14.3
 ghc-options:
   postgrest: -O2 -Werror -Wall -fwarn-identities -fno-warn-redundant-constraints -optP-Wno-nonportable-include-path
 nix: