Fix error about missing jar_installer on jbundle install

[katafrakt]

When running jruby -S jbundle install after updating to 10.0.1.0 I'm getting the following error:

LoadError: no such file to load -- jar_installer
  require at org/jruby/RubyKernel.java:1183
  require at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:136
   <main> at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/gems/shared/gems/jbundler-0.9.5/lib/jbundler/lock_down.rb:10
  require at org/jruby/RubyKernel.java:1183
  require at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:136
   <main> at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/gems/shared/gems/jbundler-0.9.5/lib/jbundler/cli.rb:24
  require at org/jruby/RubyKernel.java:1183
  require at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:136
   <main> at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/lib/ruby/gems/shared/gems/jbundler-0.9.5/bin/jbundle:46
     load at org/jruby/RubyKernel.java:1212
   <main> at /home/katafrakt/.local/share/mise/installs/ruby/jruby-10.0.1.0/bin/jbundle:25

Guessing by 5a1b151, I assumed that a similar change should be made in lock_down.rb and indeed the command runs succesfully after this change.