Use GitHub Actions for CI Builds

[jcouball]

Description

Travis builds can take an hour to start because they limit open source builds so much. See if GitHub Actions can provide the same functionality with faster feedback.

What is changed?

• The GitHub Action is defined in .github/workflows/ruby.yml. It sets up Ruby, installs dependencies, and then runs the default rake task.
• Removed .travis.yml
• Replace link to build in README.md to point to the GitHub action

What else was changed and why?

• New Rubocop offenses had to be fixed in spec/lib/ruby_git/file_helpers_spec.rb
• Moved the following GitHub specific files from the project root to the .github directory: CODEOWNERS, ISSUE_TEMPLATE.md, and PULL_REQUEST_TEMPLATE.md

[frankthrock]

Looks good, provided that the CI passes.