ruby2.5-dev 2.5.1-1ubuntu1 (i386 binary) in ubuntu cosmic

 Ruby is the interpreted scripting language for quick and easy
 object-oriented programming. It has many features to process text
 files and to do system management tasks (as in perl). It is simple,
 straight-forward, and extensible.
 .
 This package contains the header files and the mkmf library, necessary
 to make extension library for Ruby 2.5. It is also required to build
 many gems.