[Validator] Add writer to swift_version

lib/cocoapods/validator.rb

@@ -251,6 +251,10 @@ module Pod
       @swift_version ||= dot_swift_version || '2.3'
     end
 
+    # Set the SWIFT_VERSION that should be used to validate the pod.
+    #
+    attr_writer :swift_version
+
     # @return [String] the SWIFT_VERSION in the .swift-version file or nil.
     #
     def dot_swift_version

spec/unit/validator_spec.rb

@@ -834,6 +834,13 @@ module Pod
           validator.swift_version.should == '2.3'
         end
 
+        it 'allows the user to set the version' do
+          validator = test_swiftpod
+          validator.stubs(:dot_swift_version).returns('3.0')
+          validator.swift_version = '4.0'
+          validator.swift_version.should == '4.0'
+        end
+
         it 'checks for dot_swift_version' do
           validator = test_swiftpod
           validator.expects(:dot_swift_version)