Add new option to `bundle gem` for choosing a CI sevice

[colby-swandale]

Context

At the moment, every gem created with bundle gem will have configuration generated for Travis CI regardless of if you want to or not. When this change was introduced, Travis CI was a clear recommendation for most open source projects to use for testing their projects with. But this is no longer true, there are now lots of different CI services and Travis CI is no longer the clear recommendation it once was.

Changes

This PR introduces a new option to bundle gem for choosing a CI service or just not generating one at all.

Creating gem 'test'...
Do you want to add Continuous Integration to your gem? Adding a CI service to your project helps ensure your project is well tested before shipping your gem to users. Bundler recommends several different services for testing your code. For more information about each service, see:
* Travis CI:      https://travis-ci.org/
* Github Actions: https://github.com/features/actions
* Circle CI:      https://circleci.com/
* Gitlab CI:      https://docs.gitlab.com/ee/ci/

Type 'github', 'travis', 'gitlab' or 'circle' to generate those test files now and in the future. github/travis/gitlab/circle/(none):

I decided to add Github Actions, Gitlab, Circle CI along with Travis CI, which i think covers most services most people will typically go with.

Each service will generate it's own configuration which is ready to use out the box.

[colby-swandale]

@bundlerbot r+

[ghost]

Timed out

[deivid-rodriguez]

Specs seem to be hanging under Windows? Since this PR adds a new option that prompts for input, and I've never seen those hangs before, it sounds like it could be this PR's fault?