Preserve Github Flavor Markdown code block language information

Take two - https://github.com/xijo/reverse_markdown/pull/40

The previous pull request wasn't technically incorrect i.e. Github did/does recognise the highlighting when a space is included before the language identifier. However, looking at the official documentation it would seem that the preferred form is not to include a space (https://help.github.com/articles/github-flavored-markdown/).

NOTE: Compared to my previous pull request I've also swapped to stream style string concatenation, for consistency with the existing code.
Might as well be consistent with double quotes versus single quotes here.

```ruby
language_string = highlight_match ? highlight_match[1] : ""
"```" << language_string << "\n" << node.text.strip << "\n```\n"
```

By the way, if you amend the commit, you can force push the branch and it'll update the existing PR. No need to close the PR and send a new one. (You might also want to always PR a separate branch instead of the `master` branch off your fork, so that `master` isn't too tightly coupled to the PR.)