Ruby (GPLv2 only) tries to link to with readline (now GPLv3)
The readline library was relicensed under GPLv3. Since Ruby is GPLv2 only, it is not allowed to link to readline.
Most distributions currently ship both the (old) GPLv2 readline version, and the new GPLv3 readline. However, this is likely to change in the future.
A solution could be to use a different library, like libedit (http://www.thrysoee.dk/editline/)