Add Ruby 2.6 and 2.7 to CI builds
We're using 2.6 in production, and 2.7 is coming soon. Drop Ruby 2.2 and 2.3 since they are EOL'ed.
Edited by Stan Hu
We're using 2.6 in production, and 2.7 is coming soon. Drop Ruby 2.2 and 2.3 since they are EOL'ed.