Plugin dependency management revamp
This is a huge revamp of how plugin dependency management is done. To understand the changes here, a brief history lesson is in order:
Since Vagrant 1.1, plugins have been loaded as RubyGems. Once Vagrant was loaded, it would iterate through a list of installed plugins, and `require` that plugin. This mostly worked okay. But the devil is in the details, and the edge cases were _really_ bad. In addition to the edge cases (mentioned below), building things like updaters, version constraints (">= 1.0", "< 1.1"), etc. all had to be done manually. This seemed silly, since RubyGems itself (and Bundler) do these sort of things for you. Why reinvent the wheel?
As for edge cases: the primary edge case is that since the dependencies of Vagrant and its respective plugins weren't resolved as a whole, you can run into cases where plugin installation succeeded, but plugin loading failed because Vagrant already loaded a common dependency with the wrong version. An example explains this best:
* Vagrant depends on "A >= 1.0, < 1.2"
* vagrant-plugin depends on "A = 1.1"
* When you run Vagrant, it loads the latest possible matching dependencies, so it would load A 1.2
* When Vagrant loads vagrant-plugin, it can't load, because A 1.2 is active, so A 1.1 can't be loaded.
The error above should never happen: the versions available for A should satisfy both Vagrant and vagrant-plugin (by loading v1.1 for both).
With this new branch, all plugin installation, dependency resolution, updating, etc. is managed by [Bundler](http://gembundler.com). This has yielded numerous benefits:
* Vagrant now resolves dependencies before Vagrant is even loaded. This ensures that all plugins will be able to load. No more conflicts at run-time.
* Conflicts are detected at `vagrant plugin install` time. This means that if there would be a crash if that plugin were to load, the plugin won't even install and a human-friendly error is shown to the end user.
* `vagrant plugin install` now accepts complex version constraints such as "~> 1.0.0" or ">= 1.0, < 1.1". Vagrant stores these constraints for updating, which leads to the next point.
* `vagrant plugin update` without arguments now updates all installed plugins, respecting the constraints specified by `vagrant plugin install`.
* `vagrant plugin update NAME` will only update that gem (still respecting constraints).
* Internally, there are a lot more unit tests. /cc @phinze :)
The goal of this branch was to replace the _existing_ system and functionality with Bundler-ized management. It did not introduce any new features except where they naturally fell into place (version constraints). However, with this new system, many new possibilities are also available:
* Vagrant environment local plugins (i.e. a Gemfile but for a specific Vagrant environment).
* Plugin installation from git
I'm sure those will be pursued at some point in the future.
This fixes: #2612, #2406, #2428
1.1.11 was released today and seriously broke backwards compatibility.
While they never officially made any promise to follow semver, it is
almost expected at this point, but log4r decided to just fuck that.
1.1.11 changed the arity of Log4r::Logger.initialize. That seriously
breaks _everything_.
Darwin awarddddddddd goes to...
