CONTRIBUTING.md is out of date

[petergoldstein]

Expected Behaviour

CONTRIBUTING.md should reflect the current tools (GitHub Actions / Dependabot) for CI and dependency update

Current Behaviour (for problems)

The file refers to Travis and Gemnasium, which are no longer in use.

Steps to Reproduce (for problems)

Read the Code Review Process section of https://github.com/Data-Liberation-Front/csvlint.rb/blob/main/CONTRIBUTING.md

[Floppy]

Thanks for spotting that! I'm trying to bring this repo up to date and will probably miss a bunch of that sort of thing, so it's much appreciated!