install default plugins via bundler

you know, for speed.

This is an attempt to speed `rake artifact` tasks. One of the major factors slowing the process is the Gem::DependencyInstaller#install method, called for each plugin. This method will check the dependencies for the gem, meaning dependencies are checked ~100 times during the rake task.

An alternative is to hand out all the gems to bundler and have it figure out dependencies and install order in 1 go.

The approach also removes the ~100 "system" calls to "bin/plugin install" (each spawns a jvm instance).

This reduces the run time by ~60% in my tests (before: 36 minutes. now: 12 minutes).

I'm not sure if there's any negative impact on this approach, please review.

Because exec("bundle"...) looks in $PATH and ignores GEM_HOME and the current Ruby, we'll need to run the correct `bundle` if we are using the vendored JRuby.

This patch works for me for both `USE_RUBY=1` and default invocations of `rake plugin:install-defaults`

```diff
diff --git a/rakelib/plugin.rake b/rakelib/plugin.rake
index e1d1d01..4c700e7 100644
--- a/rakelib/plugin.rake
+++ b/rakelib/plugin.rake
@@ -16,6 +16,12 @@ namespace "plugin" do
       "GEM_PATH" => "#{ENV['GEM_PATH']}:vendor/bundle/jruby/1.9",
       "GEM_HOME" => "vendor/plugins/jruby/1.9"
     }
-    system(env, "bundle install")
+    if ENV['USE_RUBY'] != '1'
+      jruby = File.join("vendor", "jruby", "bin", "jruby")
+      bundle = File.join("build", "bootstrap", "bin", "bundle")
+      exec(env, jruby, "-S", bundle, "install")
+    else
+      exec(env, "bundle", "install")
+    end
   end
 end # namespace "plugin"
diff --git a/rakelib/z_rubycheck.rake b/rakelib/z_rubycheck.rake
index 3b7a9f0..a48b0d4 100644
--- a/rakelib/z_rubycheck.rake
+++ b/rakelib/z_rubycheck.rake
@@ -1,6 +1,6 @@
 if ENV['USE_RUBY'] != '1'
   if RUBY_ENGINE != "jruby" or Gem.ruby !~ /vendor\/jruby\/bin\/jruby/
-    puts "Restarting myself under Vendored JRuby (currently #{RUBY_ENGINE} #{RUBY_VERSION})"  if $DEBUG
+    puts "Restarting myself under Vendored JRuby (currently #{RUBY_ENGINE} #{RUBY_VERSION})"  if ENV["DEBUG"]

     # Make sure we have JRuby, then rerun ourselves under jruby.
     Rake::Task["vendor:jruby"].invoke
```

The bug fixed is that if `ruby` is MRI, `rake plugin:install-defaults` currently (without this patch) runs `bundle install` which runs under MRI on my workstation.