Drop support for deprecated Ruby versions

[tancnle]

Description

With Ruby 2.4 EOL (https://www.ruby-lang.org/en/news/2020/04/05/support-of-ruby-2-4-has-ended/), we should drop support for old versions to reduce Rouge gem maintenance effort. It also helps to reduce build runtime and avoid overrunning our monthly CI build time allotment.

Proposal

• Update README and gemspec to indicate this change
• Remove Ruby 2.4 and below from CI pipelines (i.e. .travis.yml)
• Update Rubocop to target Ruby 2.5 (i.e. rubocop.yml)
• Mark this change for a minor version bump

[tancnle]

/cc @dblessing as discussed.

[dblessing]

Thanks @tancnle. I agree with only supporting 2.5+

[stale]

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[tancnle]

Resolved by #1862