require newer asciidoc (>=0.1.4) in order to match test data

Asciidoc versions prior to 0.1.4 would write HTML output with a couple of extra newline characters, and this would cause the tests to fail. Ensure that we're using the very latest version of Asciidoc to avoid this.

```
  1) Failure:
MarkupTest#test_asciidoc
[/builddir/build/BUILD/github-markup-1.0.0/usr/share/ge
ms/gems/github-markup-1.0.0/test/markup_test.rb:28]:
README.asciidoc.html's contents don't match command output:
--- -   2013-12-30 09:20:16.824751950 -0700
+++ test/markups/README.asciidoc.html   2013-12-30 09:20:15.464500241
-0700
@@ -1,5 +1,4 @@
 <div class="ulist">
-
 <ul>
 <li>
 <p>One</p>
@@ -8,4 +7,4 @@
 <p>Two</p>
 </li>
 </ul>
-</div>
\ No newline at end of file
+</div>
```