use gem/rip versions of deps instead of vendoring

lib/middleman/sprockets+ruby19.rb

@@ -0,0 +1,29 @@
+module Sprockets
+  class SourceFile
+    def source_lines
+      @lines ||= begin
+        lines = []
+
+        comments = []
+        File.open(pathname.absolute_location, 'rb') do |file|
+          file.each do |line|
+            lines << line = SourceLine.new(self, line, file.lineno)
+
+            if line.begins_pdoc_comment? || comments.any?
+              comments << line
+            end
+
+            if line.ends_multiline_comment?
+              if line.ends_pdoc_comment?
+                comments.each { |l| l.comment! }
+              end
+              comments.clear
+            end
+          end
+        end
+
+        lines
+      end
+    end
+  end
+end