Merge branch 'hotfix/1.23.3'
fix selectig download binaries for remote jruby-head, update travis-ci/travis-rubies#1
handle jruby-head as a special case when downloading binary, fix travis-ci/travis-rubies#1
simplify gem installation
fix detecting jruby native directories
make sure gemset/bin is created on gemset initialization
do not show deprecation warning for head ruby, fix #2273
1.23.4
Merge branch 'master' into hotfix/1.23.4
Merge branch 'hotfix/1.23.4' into stable