Fix JRuby installation

* #{bin_dir}/gem requires #{bin_dir}/jruby in PATH

* instead of executing `gem list` for bundler we check
  if bundler bin exists. Because our check would fail due to
  the same reason as `gem install bundler` mentioned above :)

* optionally set JAVA_HOME. On my Vagrant machine it works without
  setting it.

* I also tried thing such as `. #{profile_file} && gem install` but
  decided not to use such approach because profile_file might have
  another rules inside that we are not interested in.

fix #4