ruby_targets options

rbx
Build with Rubinius
ruby21
Build with MRI Ruby 2.1.x
ruby22
Build with MRI Ruby 2.2.x
ruby23
Build with MRI Ruby 2.3.x
ruby24
Build with MRI Ruby 2.4.x
ruby25
Build with MRI Ruby 2.5.x
ruby26
Build with MRI Ruby 2.6.x