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.

Nice. though the problem in using exec is that it replaces the current process with the bundler execution, thus stopping the parent `rake artifact:*` task. Using system instead fixes this.

I'm now able to run `rake artifact:rpm` once, when I run it again it breaks, complaining about missing `cabin` gem..Debugging..