Upgrade to Ruby v2.7.2 in CI

Since Ruby 2.7.2 comes with bundler v2.1.4 by default, we need to
upgrade the Gemfile.lock and bump the CI cache keys. We also need to
install bundler v1.17.3 to ensure a smooth transition for other
dependencies, like Gitaly.

Exclude development gems from QA bundle install since debaser doesn't
compile on Ruby 2.7.