don't let script fail if file doesn't exist

sometimes for me the rm command in this script fails if the file
doesn't exist. I'm not sure why that would happen, but using the -f
flag on rm will prevent this command from failing and halting a build.

lib/cocoapods/generator/copy_resources_script.rb

@@ -117,7 +117,7 @@ EOS
       RSYNC_CALL = <<EOS
 
 rsync -avr --no-relative --exclude '*/.svn/*' --files-from="$RESOURCES_TO_COPY" / "${CONFIGURATION_BUILD_DIR}/${UNLOCALIZED_RESOURCES_FOLDER_PATH}"
-rm "$RESOURCES_TO_COPY"
+rm -f "$RESOURCES_TO_COPY"
 EOS
 
     end