Upgrade to Ruby 2.6.3
Ruby 2.6.3 offers better performance and more efficient memory usage. We use Ruby 2.6.3 for CE and EE, so we ought to make the version consistent so users don't need to have multiple versions for no real reason. In addition, Middleman 5 is going to need Ruby 2.5+ for https://github.com/middleman/middleman/pull/2276.
Edited by Stan Hu