Avoid modification on possibly frozen string
Avoid modification on possibly frozen string

Most users are defining option description with string literal. Starting from ruby 2.3, rubocop starts advocating to set `# frozen_string_literal: true` magic comment.

Before this patch, this would raise an error related to modification of frozen string.

Change-Id: Ia2561296a5855e639a4972368ab00ccbccd7cb12
Signed-off-by: Grégoire Seux <139507733e92493cefa400d92b851cc35cdd02bf@criteo.com>

chef/mixlib-cli,chef/mixlib-cli