Refactor build tooling to use Ruby

This is for #1640

Major changes:
* rake is used instead of make. This allows us to more easily improve,
  debug, and test the build tooling.
* The build process can be bootstrapped from any Ruby that has
  Rake available. Bootstrapping rake is automatic will download JRuby and
  re-execute itself under JRuby.

Internal changes:
* Vendoring paths are project-based. Each vendor directory is now
  specific as vendor/<project>/... For example, JRuby is dumped into
  vendor/jruby/...

Other changes:

* We don't use jruby-complete anymore. We now use bin/jruby as the entry point.
  This improves the startup time of Logstash on my laptop by approximately
  1.5 seconds (~25% speedup of startup time). Testing `bin/logstash
  version` speed; 5 runs each:

  * Before patch: 7.27 7.55 7.26 7.42 7.47
  * After patch: 5.54 5.97 5.67 5.58 5.54

Smaller changes:
- removed 'env' subcommand
- Add gem_home method for this platform's GEM_HOME