[RuboCop] Enable Style/DoubleNegation

.rubocop_todo.yml

@@ -27,10 +27,6 @@ Metrics/PerceivedComplexity:
 Style/CommentAnnotation:
   Enabled: false
 
-# Offense count: 1
-Style/DoubleNegation:
-  Enabled: false
-
 # Offense count: 6
 # Configuration parameters: AllowedVariables.
 Style/GlobalVars:

lib/cocoapods/installer/analyzer.rb

@@ -101,7 +101,7 @@ module Pod
       #         change in the Podfile should be locked.
       #
       def update_mode?
-        !!update
+        update != nil
       end
 
       # @return [Symbol] Whether and how the dependencies in the Podfile

spec/unit/validator_spec.rb

@@ -322,7 +322,9 @@ module Pod
         it 'includes the use_frameworks!(false) directive' do
           podfile = @validator.send(:podfile_from_spec, :ios, '5.0', false)
           target_definition = podfile.target_definitions['Pods']
+          # rubocop:disable Style/DoubleNegation
           (!!target_definition.uses_frameworks?).should == false
+          # rubocop:enable Style/DoubleNegation
         end
       end