Bugfix, call horizRule with proper context

It was a bug, when gruber dialect call 'lists' method and 'lists' call horizRule but with wrong context. Thus I had an error in browser. Here are its text and stack:

Uncaught TypeError: Object #<Object> has no method 'processBlock' markdown.js:398
horizRule markdown.js:398
Markdown.dialects.Gruber.block.lists markdown.js:651
processBlock markdown.js:226
toTree markdown.js:261
expose.parse markdown.js:72
toHTMLTree markdown.js:103
toHTML