Fix `vagrant plugin` commands

`StateFile#installed_plugins` returns now a Hash instead of Array.

Fixes regression from 39b2539.

plugins/commands/plugin/action/license_plugin.rb

@@ -18,7 +18,7 @@ module VagrantPlugins
 
         def call(env)
           # Get the list of installed plugins according to the state file
-          installed = Set.new(env[:plugin_state_file].installed_plugins)
+          installed = env[:plugin_state_file].installed_plugins.keys
 
           # If the plugin we're trying to license doesn't exist in the
           # state file, then it is an error.

plugins/commands/plugin/action/list_plugins.rb

@@ -18,7 +18,7 @@ module VagrantPlugins
 
         def call(env)
           # Get the list of installed plugins according to the state file
-          installed = Set.new(env[:plugin_state_file].installed_plugins)
+          installed = env[:plugin_state_file].installed_plugins.keys
 
           # Go through the plugins installed in this environment and
           # get the latest version of each.

plugins/commands/plugin/action/plugin_exists_check.rb

@@ -12,7 +12,7 @@ module VagrantPlugins
 
         def call(env)
           # Get the list of installed plugins according to the state file
-          installed = Set.new(env[:plugin_state_file].installed_plugins)
+          installed = env[:plugin_state_file].installed_plugins.keys
           if !installed.include?(env[:plugin_name])
             raise Vagrant::Errors::PluginNotInstalled,
               name: env[:plugin_name]

plugins/commands/plugin/action/prune_gems.rb

@@ -34,7 +34,7 @@ module VagrantPlugins
           @logger.info("Pruning gems...")
 
           # Get the list of installed plugins according to the state file
-          installed = Set.new(env[:plugin_state_file].installed_plugins)
+          installed = env[:plugin_state_file].installed_plugins.keys
 
           # Get the actual specifications of installed gems
           all_specs = env[:gem_helper].with_environment do

plugins/commands/plugin/state_file.rb

@@ -32,11 +32,11 @@ module VagrantPlugins
         save!
       end
 
-      # This returns a list of installed plugins according to the state
+      # This returns a hash of installed plugins according to the state
       # file. Note that this may _not_ directly match over to actually
       # installed gems.
       #
-      # @return [Array<String>]
+      # @return [Hash]
       def installed_plugins
         @data["installed"]
       end