Fix copy resources script for iOS < 6 and OS X < 10.8

Remove the following flag from the script to avoid crashes

```
man ibtool:

--reference-external-strings-file
              When combined with the --compile option, this flag indicates that the files in the Base.lproj locale  folder  should  be
              compiled  to reference the matching external strings files found in the other locales when loaded. Available on 10.8 and
              later.
```

New to cocoa pods (and Ruby!) so **I need some help to evaluate `greaterorquealto_osx10_8_or_ios6 `**:

``` ruby
# target_installer.rb

      ENABLE_EXTERNAL_STRINGS_FILE_FLAG = {
        :ios => Version.new('6'),
        :osx => Version.new('10.8')
      }

      def create_copy_resources_script
        path = library.copy_resources_script_path
        UI.message "- Generating copy resources script at #{UI.path(path)}" do
          resources = library.file_accessors.map { |accessor| accessor.resources.flatten.map {|res| project.relativize(res)} }.flatten
          resources << bridge_support_file if bridge_support_file
          platform_name = library.platform.name
          reference_external_strings_file = library.platform.deployment_target >= ENABLE_EXTERNAL_STRINGS_FILE_FLAG[platform_name]
          generator = Generator::CopyResourcesScript.new(resources, reference_external_strings_file)
          generator.save_as(path)
          add_file_to_support_group(path)
        end
      end
```

Which in turn will remove the offending `--reference-external-strings-file` flag:

``` ruby
# copy_resources_script.rb

      def save_as(pathname)
        pathname.open('w') do |script|
          script.puts @reference_external_strings_file ? CONTENT : CONTENT.gsub(' --reference-external-strings-file', '')
          @resources.each do |resource|
            script.puts "install_resource '#{resource}'"
          end
        end
        # @todo use File api
        system("chmod +x '#{pathname}'")
      end
```