I am not sure what to recommend on MacOS but the homebrew jenv route looks sensible. On debian linux I use update-alternatives (that uses symbolic links to manage versions), and if you check jruby version:-
jruby --version
you get when you change to java-11
jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 Java HotSpot(TM) 64-Bit Server VM 11.0.1+13-LTS on 11.0.1+13-LTS +jit [linux-x86_64]
and
jruby 9.2.0.0 (2.5.0) 2018-05-24 81156a8 OpenJDK 64-Bit Server VM 25.181-b13 on 1.8.0_181-8u181-b13-0ubuntu0.18.04.1-b13 +jit [linux-x86_64]
When you switch back with sudo update-alternatives --config java and it is fairly painless.