Do not catch USR1 on JRuby

Attempting to catch USR1 on the JVM raises the following message:

> The signal USR1 is in use by the JVM and will not work correctly on this
> platform
Merge pull request #426 from nanoc/fix/disable-usr1-on-jruby

Do not catch USR1 on JRuby
Merge branch 'release-3.6.x'