Use bundler to load plugins
warn if running in a bundler env, don't load plugins
commands/plugin: allow within Bundler env (cause we always are)
core: Vagrant.require_plugin is gone
Move StateFile into Vagrant::Plugin core
commands/plugin: list plugins uses Bundler
commands/plugin: remove unnecessary line
commands/plugin: list uses PluginManager
installing gems works
commands/plugin: Remove PruneGems
load with bundler context
fast path Vagrant --version
Lock to log4r < 1.1.11 because we got fucked

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...
fast-path vagrant --version
Put plugins in their own Gemfile group so we can load on their own
commands/plugin: uninstall works again
commands/plugin: error if uninstall non-existent plugin
Don't load plugins again on `vagrant plugin` commands
Work some things around so that Bundler is not setup with no plugins
Lower some branching logic