make compatible with 64 bit

lib/rbnacl/libsodium.rb

-require "rbnacl/libsodium/version"
+require 'rbnacl/libsodium/version'
 
 module RbNaCl
   module Libsodium
-    sodiumlib_dir = File.expand_path("../../../vendor/libsodium/dist/lib/", __FILE__)
+    class << self
+      def sodiumlib_dir
+        sodiumlib32_dir = File.expand_path('../../../vendor/libsodium/dist/lib/', __FILE__)
+        sodiumlib64_dir = File.expand_path('../../../vendor/libsodium/dist/lib64/', __FILE__)
+        [sodiumlib32_dir, sodiumlib64_dir].select { |dir| Dir.exist?(dir) }.first
+      end
 
-    sodiumlib_glob = case RUBY_DESCRIPTION
-    when /darwin/ then "libsodium*.dylib"
-    when /Windows|(win|mingw)32/ then "libsodium*.dll"
-    when /openbsd/ then "libsodium*.so.*"
-    else "libsodium*.so"
+      def sodiumlib_glob
+        case RUBY_DESCRIPTION
+        when /darwin/ then 'libsodium*.dylib'
+        when /Windows|(win|mingw)32/ then 'libsodium*.dll'
+        when /openbsd/ then 'libsodium*.so.*'
+        else 'libsodium*.so'
+        end
+      end
     end
 
     ::RBNACL_LIBSODIUM_GEM_LIB_PATH = Dir.glob(File.join(sodiumlib_dir, sodiumlib_glob)).first
   end
 end
 
-require "rbnacl"
+require 'rbnacl'