reference_proxy: Remove build files that refer to it when removed.

A reference proxy might have build files that refer to it, and as
mentioned in the previous commit, they don't have any meaning without
it. Therefore they are removed when the reference proxy is removed.

lib/xcodeproj/project/object/reference_proxy.rb

@@ -61,6 +61,25 @@ def display_name
           return path if path
           super
         end
+
+        # @return [Array<PBXBuildFile>] the build files associated with the
+        #         current reference proxy.
+        #
+        def build_files
+          referrers.grep(PBXBuildFile)
+        end
+
+        # In addition to removing the reference proxy, this will also remove any
+        # items related to this reference.
+        #
+        # @see AbstractObject#remove_from_project
+        #
+        # @return [void]
+        #
+        def remove_from_project
+          build_files.each(&:remove_from_project)
+          super
+        end
       end
     end
   end

spec/project/object/reference_proxy_spec.rb

@@ -24,5 +24,13 @@ module ProjectSpecs
       @proxy.path = 'Path/To/Proxy'
       @proxy.display_name.should == 'Path/To/Proxy'
     end
+
+    it 'removes the build files when removing the reference proxy' do
+      @target = @project.new_target(:static_library, 'Pods', :ios)
+      @target.build_phases[0].add_file_reference(@proxy)
+
+      @proxy.remove_from_project
+      @target.build_phases[0].files.should.be.empty
+    end
   end
 end