commands/plugin: remove --plugin-prerelease

plugins/commands/plugin/action/install_gem.rb

@@ -23,7 +23,6 @@ module VagrantPlugins
 
         def call(env)
           plugin_name = env[:plugin_name]
-          prerelease  = env[:plugin_prerelease]
           version     = env[:plugin_version]
 
           # Determine the plugin name we'll look for in the installed set
@@ -46,7 +45,6 @@ module VagrantPlugins
 
           # Install the gem
           plugin_name_label = plugin_name
-          plugin_name_label += ' --prerelease' if prerelease
           plugin_name_label += " --version '#{version}'" if version
           env[:ui].info(I18n.t("vagrant.commands.plugin.installing",
                                :name => plugin_name_label))

plugins/commands/plugin/command/install.rb

@@ -26,7 +26,6 @@ module VagrantPlugins
           # Install the gem
           action(Action.action_install, {
             :plugin_entry_point => options[:entry_point],
-            :plugin_prerelease  => options[:plugin_prerelease],
             :plugin_version     => options[:plugin_version],
             :plugin_sources     => options[:plugin_sources],
             :plugin_name        => argv[0]

plugins/commands/plugin/command/mixin_install_opts.rb

@@ -8,9 +8,13 @@ module VagrantPlugins
             options[:entry_point] = entry_point
           end
 
+          # @deprecated
           o.on("--plugin-prerelease",
                "Allow prerelease versions of this plugin.") do |plugin_prerelease|
-            options[:plugin_prerelease] = plugin_prerelease
+            puts "--plugin-prelease is deprecated and will be removed in the next"
+            puts "version of Vagrant. It has no effect now. Use the '--plugin-version'"
+            puts "flag to get a specific pre-release version."
+            puts
           end
 
           o.on("--plugin-source PLUGIN_SOURCE", String,