Adapt plugin loading for Vagrant 1.5

`Vagrant.require_plugin` will be deprecated in Vagrant 1.5.

In new Vagrant versions we now use `require` directly for loading the
optional dependencies. All exceptions are logged but ignored. Vagrant
itself will handle the real errors later (or has already errored out).

lib/vagrant-proxyconf/plugin.rb

@@ -1,4 +1,5 @@
 require 'vagrant'
+require_relative 'logger'
 
 module VagrantPlugins
   module ProxyConf
@@ -42,19 +43,36 @@ def self.setup_i18n
       end
 
       # Ensures a dependent plugin is loaded before us if it is installed.
-      # Ignores {Vagrant::Errors::PluginLoadError} but passes other exceptions.
+      # Ignores Errors while loading, as Vagrant itself anyway shows them to
+      # used when *it* tries to load the plugin.
       #
       # @param plugin [String] the plugin name
       def self.load_optional_dependency(plugin)
-        begin
-          Vagrant.require_plugin plugin
-        rescue Vagrant::Errors::PluginLoadError; end
+        logger = ProxyConf.logger
+        logger.info "Trying to load #{plugin}"
+
+        if check_vagrant_version('< 1.5.0.dev')
+          begin
+            Vagrant.require_plugin plugin
+          rescue Vagrant::Errors::PluginLoadError
+            logger.info "Ignoring the load error of #{plugin}"
+          end
+        else
+          begin
+            require plugin
+          rescue Exception => e
+            logger.info "Failed to load #{plugin}: #{e.inspect}"
+            logger.info "Ignoring the error"
+          end
+        end
       end
 
       # Loads the plugins to ensure their action hooks are registered before us.
       # Uses alphabetical order to not change the default behaviour otherwise.
       def self.load_optional_dependencies
-        OPTIONAL_PLUGIN_DEPENDENCIES.sort.each { |plugin| load_optional_dependency plugin }
+        OPTIONAL_PLUGIN_DEPENDENCIES.sort.each do |plugin|
+          load_optional_dependency plugin
+        end
       end
 
       setup_i18n

spec/unit/vagrant-proxyconf/plugin_spec.rb

@@ -67,19 +67,35 @@
     subject { described_class.load_optional_dependency(plugin_name) }
     let(:plugin_name) { 'vagrant-foo' }
 
-    it "loads the specified plugin" do
-      expect(Vagrant).to receive(:require_plugin).with(plugin_name)
-      subject
-    end
+    # Vagrant plugin loading API changed in v1.5.0
+    if Gem::Version.new(Vagrant::VERSION) < Gem::Version.new('1.5.0.dev')
+      it "loads the specified plugin" do
+        expect(Vagrant).to receive(:require_plugin).with(plugin_name)
+        subject
+      end
 
-    it "ignores PluginLoadError" do
-      expect(Vagrant).to receive(:require_plugin).and_raise(Vagrant::Errors::PluginLoadError, plugin: plugin_name)
-      expect { subject }.not_to raise_error
-    end
+      it "ignores PluginLoadError" do
+        expect(Vagrant).to receive(:require_plugin).
+          and_raise(Vagrant::Errors::PluginLoadError, plugin: plugin_name)
+        expect { subject }.not_to raise_error
+      end
 
-    it "won't ignore other error" do
-      expect(Vagrant).to receive(:require_plugin).and_raise(Vagrant::Errors::PluginLoadFailed, plugin: plugin_name)
-      expect { subject }.to raise_error(Vagrant::Errors::PluginLoadFailed)
+      it "won't ignore other error" do
+        expect(Vagrant).to receive(:require_plugin).
+          and_raise(Vagrant::Errors::PluginLoadFailed, plugin: plugin_name)
+        expect { subject }.to raise_error(Vagrant::Errors::PluginLoadFailed)
+      end
+    else
+      it "loads the specified plugin" do
+        expect(described_class).to receive(:require).with(plugin_name)
+        subject
+      end
+
+      it "ignores errors" do
+        expect(described_class).to receive(:require).
+          and_raise(LoadError, path: plugin_name)
+        expect { subject }.not_to raise_error
+      end
     end
   end