Fixed issues for 764823b84d6bd62d37623cbed62809649f0c60c7

* Change the indicator for code block to ````` instead of `<pre` tag to be
consistent with Markdown.
* Fixed the RegEx to capture all code blocks instead of bundling all of
them into one.
* Fixed issue with cache when updating a gist in which there was no
cache for the gists list causing a `bad_match`.
* Fixed some tests.