Initialize silence_spec_warnings

Signed-off-by: Michael Koziarski <michael@koziarski.com>
[#1128 state:resolved]

railties/lib/rails/vendor_gem_source_index.rb

@@ -13,6 +13,8 @@ module Rails
     attr_reader :installed_source_index
     attr_reader :vendor_source_index
 
+    @@silence_spec_warnings = false
+
     def self.silence_spec_warnings
       @@silence_spec_warnings
     end