[HooksManager] Show the file that a hook without a name comes from

Closes https://github.com/CocoaPods/CocoaPods/issues/2990.

lib/cocoapods/hooks_manager.rb

@@ -45,7 +45,7 @@ module Pod
         raise ArgumentError, 'Missing block' unless block
 
         UI.warn '[Hooks] The use of hooks without specifying a `plugin_name` ' \
-                'has been deprecated.' unless plugin_name
+                "has been deprecated (from file `#{block.binding.eval('File.expand_path __FILE__')}`)." unless plugin_name
 
         @name = name
         @plugin_name = plugin_name

spec/unit/hooks_manager_spec.rb

@@ -32,6 +32,7 @@ module Pod
       it 'warns if no plugin name is given' do
         @hooks_manager.register(:post_install) {}
         UI.warnings.should.match /hooks without.*deprecated/
+        UI.warnings.should.match /#{__FILE__}/
       end
     end