replace Dir.mkdir with FileUtils.mkdir_p for recursive creation of nonexistent directories

lib/squib/args/dir_validator.rb

@@ -6,7 +6,7 @@ module Squib
       def ensure_dir_created(dir)
         unless Dir.exists?(dir)
           Squib.logger.warn "Dir '#{dir}' does not exist, creating it."
-          Dir.mkdir dir
+          FileUtils.mkdir_p dir
         end
         return dir
       end

lib/squib/card.rb

@@ -34,7 +34,7 @@ module Squib
       when :memory
         Cairo::ImageSurface.new(@width, @height)
       when :svg
-        Dir.mkdir @deck.dir unless Dir.exists?(@deck.dir)
+        FileUtils.mkdir_p @deck.dir unless Dir.exists?(@deck.dir)
         Cairo::SVGSurface.new(svgfile, @width, @height)
       else
         Squib.logger.fatal "Back end not recognized: '#{backend}'"