Add support for Ruby 3.0.0

[pblesi]

This addresses issue: https://github.com/braintree/runbook/issues/40

In addition to adding support for Ruby 3.0.0, this commit does the following:

• Adds support for Ruby 2.7.2
• Removes support for Ruby 2.2.10
• Adds support for Bundler < 3.0
• Drops support for Bundler < 2.2.0
• Adds a ci option to the project generator

[coopergillan]

@pblesi - this one now needs a merge conflict resolution due to gem upgrades.