vici: Use default Unix vici socket if none passed to ruby constructor

While we currently have a static path instead of one generated with Autotools,
this at least is congruent to what we have in the Python library.

src/libcharon/plugins/vici/README.md

@@ -821,9 +821,9 @@ during encoding.
 
 ## Connecting to the daemon ##
 
-To create a connection to the daemon, a socket must be passed to the
-_Connection_ constructor. There is no default, but on Unix systems usually
-a Unix socket over _/var/run/charon.vici_ is used:
+To create a connection to the daemon, a socket can be passed to the
+_Connection_ constructor. If none is passed, a default Unix socket at
+_/var/run/charon.vici_ is used:
 
 	require "vici"
 	require "socket"

src/libcharon/plugins/vici/ruby/lib/vici.rb

@@ -390,7 +390,10 @@ module Vici
   # during encoding.
   class Connection
 
-    def initialize(socket)
+    def initialize(socket = nil)
+      if socket == nil
+        socket = UNIXSocket.new("/var/run/charon.vici")
+      end
       @transp = Transport.new(socket)
     end