improve robustness of gem check

[Alessandro-Barbieri]

If you have multiple versions of the same gem installed (for example json 2.5.1 and 2.6.1)
this will happen

checking ruby gem: json... yes (detected: 2.6.1, 2.3.0)
./configure: line 7805: 2.3.0: command not found

See https://bugs.gentoo.org/840478

[tomjelinek]

Thanks for reporting this issue!