Ruby uses semver after 2.1.0.

# .travis.yml
- 1.9.2
- 1.9.3
- 2.0.0
- 2.1
- 2.2

So this means 1.9.2-p330(latest), 1.9.3-p551(latest), 2.0.0-p643(latest), 2.1.5(latest), 2.2.1(latest). But 2.1.0 means exactly 2.1.0.

They follows Travis-CI’s latest precompiled one.

• Ruby version policy changes starting with Ruby 2.1.0
• ruby-build’s targets
• Travis CI: Precompiled Ruby Versions