Refactor build tooling to use Ruby

The new way to build is using 'rake' and not 'make'.

For some some attempt at compatibility, any 'make' invocation will
invoke rake. `make foo` will invoke `rake foo`.

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
- .travis.yml removed because we haven't used Travis in a long time.
- bin/logstash exits nonzero with a useful message if JRuby can't be found.
Invoke bundler via system() call.

After some discussion in #bundler on freenode (very helpful!) on how
to do what we need to do with bundler, I was encouraged to not use the
bundler api from ruby, so system() it is.