make verify-codegen is failing on the master branch

[ian-howell]

Is this a BUG REPORT or FEATURE REQUEST?:
BUG REPORT

What happened:
I've just created a Pull Request, but the make verify-codegen target is failing due to the following error:

./hack/verify-codegen.sh
Generating deepcopy funcs
Generating clientset for workflow:v1alpha1 at github.com/argoproj/argo/pkg/client/clientset
Generating listers for workflow:v1alpha1 at github.com/argoproj/argo/pkg/client/listers
Generating informers for workflow:v1alpha1 at github.com/argoproj/argo/pkg/client/informers
diffing ./hack/../pkg against freshly generated codegen
./hack/../pkg up to date.
./hack/update-openapigen.sh --verify-only
mkdir -p /home/ian/gocode/src/github.com/argoproj/argo/dist
go run ./hack/gen-openapi-spec/main.go 2.4.0 > /home/ian/gocode/src/github.com/argoproj/argo/dist/swagger.json
diff /home/ian/gocode/src/github.com/argoproj/argo/dist/swagger.json /home/ian/gocode/src/github.com/argoproj/argo/api/openapi-spec/swagger.json
5c5
<     "version": "v2.4.0"
---
>     "version": "v2.3.0-rc3"
Makefile:146: recipe for target 'verify-codegen' failed
make: *** [verify-codegen] Error 1

I thought this looked strange, since my change shouldn't have broken in that way, so I rolled back the change and reran make verify-codegen from the tip of master. Sure enough, the target is still failing. This prevents any future PR's from passing the checks.

How to reproduce it (as minimally and precisely as possible):
clone argo and run make verify-codegen.

Environment:

• Argo version:

$ ./dist/argo version
argo: v2.4.0+270aabf.dirty
  BuildDate: 2019-06-05T20:53:26Z
  GitCommit: 270aabf1d8cabd69b9851209ad5d9c874348e21d
  GitTreeState: clean
  GoVersion: go1.12
  Compiler: gc
  Platform: linux/amd64

[jessesuen]

Fixed