Update graphql minimum version

[Samsinite]

I'm not certain, but I believe the visitor changes require graphql 2.1.0+, so here is a PR to bump the gem spec requirements, though @rmosolgo you'll likely know for certain if this needs changed or not.

Btw, thanks for taking the initiative and getting updates to this library rolling.

[rmosolgo]

I thought we ran CI on a bunch of GraphQL versions ... but now I see they're all 2.1.0 😆

I would expect my change to work with any GraphQL-Ruby version after 1.9.0; the class-based visitor API which is used by that change was added in rmosolgo/graphql-ruby#1290

I don't want to cut off people who are using older GraphQL-Ruby versions if I can help it, so I'd rather confirm this constraint in practice before we publish it. Would you be game to modify our CI setup to run at least GraphQL-Ruby 1.12.x, 1.13.x, 2.0.x, 2.1.x, and 2.2.x? IMO we could apply those versions to some of the jobs that already run.

[Samsinite]

I thought we ran CI on a bunch of GraphQL versions ... but now I see they're all 2.1.0 😆

I would expect my change to work with any GraphQL-Ruby version after 1.9.0; the class-based visitor API which is used by that change was added in rmosolgo/graphql-ruby#1290

I don't want to cut off people who are using older GraphQL-Ruby versions if I can help it, so I'd rather confirm this constraint in practice before we publish it. Would you be game to modify our CI setup to run at least GraphQL-Ruby 1.12.x, 1.13.x, 2.0.x, 2.1.x, and 2.2.x? IMO we could apply those versions to some of the jobs that already run.

I'll give it a try later this week and see what I can come up with, and if successful I'll include the minimum working versions in the gemspec :D.