swagger-codegen: 2.2.1 -> 2.3.1

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/x901ajf5y7i8xg81lnngw7p9fwyn73da-swagger-codegen-2.3.1/bin/swagger-codegen help` got 0 exit code
- ran `/nix/store/x901ajf5y7i8xg81lnngw7p9fwyn73da-swagger-codegen-2.3.1/bin/swagger-codegen version` and found version 2.3.1
- found 2.3.1 with grep in /nix/store/x901ajf5y7i8xg81lnngw7p9fwyn73da-swagger-codegen-2.3.1
- found 2.3.1 in filename of file in /nix/store/x901ajf5y7i8xg81lnngw7p9fwyn73da-swagger-codegen-2.3.1

cc "@jraygauthier"