This fixes test failure with html-pipeline 2.12

https://github.com/github/markup/issues/1317

--- a/test/markups/README.org.html
+++ b/test/markups/README.org.html
@@ -13,14 +13,17 @@
 <td>0.9.0</td>
 </tr>
 </table>
-<h1>1 Description</h1>
+<h1>
+<span>1</span> Description</h1>
 <p>Helpful Ruby routines for parsing orgmode files. The most
   significant thing this library does today is convert orgmode files
   to textile. Currently, you cannot do much to customize the
   conversion. The supplied textile conversion is optimized for
   extracting “content” from the orgfile as opposed to “metadata.”</p>
-<h1>2 History</h1>
-<h2>2.1 2014-02-08: Version 0.9.0</h2>
+<h1>
+<span>2</span> History</h1>
+<h2>
+<span>2.1</span> 2014-02-08: Version 0.9.0</h2>
 <ul>
   <li>Let’s make sure <code>#+INCLUDE:</code> is not supported</li>
 </ul>
@@ -35,13 +38,15 @@
   end
 end
 </pre>
-<h2>2.2 2009-12-30: Version 0.5.1</h2>
+<h2>
+<span>2.2</span> 2009-12-30: Version 0.5.1</h2>
 <ul>
   <li>Minor enhancement: Recognize lines starting with “:” as examples.</li>
   <li>Minor enhancement: Recognize #+BEGIN_SRC as source blocks</li>
   <li>Minor enhancement: Add “src” and “example” classes to &lt;pre&gt; blocks.</li>
 </ul>
-<h2>2.3 2009-12-30: Version 0.5.0</h2>
+<h2>
+<span>2.3</span> 2009-12-30: Version 0.5.0</h2>
 <ul>
   <li>Parse (but not necessarily <b>use</b>) in-buffer settings. The following
     in-buffer settings <b>are</b> used:
@@ -65,7 +70,8 @@
     </ul>
   </li>
 </ul>
-<h2>2.4 2009-12-29: Version 0.4.2</h2>
+<h2>
+<span>2.4</span> 2009-12-29: Version 0.4.2</h2>
 <ul>
   <li>Got rid of the extraneous newline at the start of code blocks.</li>
   <li>Everything now shows up in code blocks, even org-mode metadata.</li>
@@ -76,12 +82,14 @@
     </ul>
   </li>
 </ul>
-<h2>2.5 2009-12-29: Version 0.4.1</h2>
+<h2>
+<span>2.5</span> 2009-12-29: Version 0.4.1</h2>
 <ul>
   <li>HTML is now escaped by default</li>
   <li>org-mode comments will show up in a code block.</li>
 </ul>
-<h2>2.6 2009-12-29: Version 0.4</h2>
+<h2>
+<span>2.6</span> 2009-12-29: Version 0.4</h2>
 <ul>
   <li>The first thing output in HTML gets the class “title”</li>
   <li>HTML output is now indented</li>
@@ -94,7 +102,8 @@
     </ul>
   </li>
 </ul>
-<h2>2.7 2009-12-27: Version 0.3</h2>
+<h2>
+<span>2.7</span> 2009-12-27: Version 0.3</h2>
 <ul>
   <li>Uses rubypants to get better typography (smart quotes, ellipses, etc…).</li>
   <li>Fixed bugs:
@@ -105,14 +114,16 @@
     </ul>
   </li>
 </ul>
-<h2>2.8 2009-12-26: Version 0.2</h2>
+<h2>
+<span>2.8</span> 2009-12-26: Version 0.2</h2>
 <ul>
   <li>Added <code>to_html</code> output on the parser.</li>
   <li>Added support for the full range of inline markup: <b>bold</b>,
-    <i>italic</i>, <code>code</code>, <code>verbatim</code>, underline, <del>strikethrough</del>.</li>
+    <i>italic</i>, <code>code</code>, <code>verbatim</code>, <span>underline</span>, <del>strikethrough</del>.</li>
   <li>Lots of refactoring to make the code more maintainable.</li>
 </ul>
-<h2>2.9 2009-12-23: Version 0.1</h2>
+<h2>
+<span>2.9</span> 2009-12-23: Version 0.1</h2>
 <ul>
   <li>Added support for block code, like this:
     <pre>
@@ -136,4 +147,4 @@
     greatly simplified a lot of the messiness of <code>textile</code>
     conversion.</li>
   <li>Added support for line breaks within list items.</li>
-</ul>
\ No newline at end of file
+</ul>
--- a/github-markup.gemspec
+++ b/github-markup.gemspec
@@ -19,7 +19,7 @@
   s.add_development_dependency 'rake', '~> 12'
   s.add_development_dependency 'activesupport', '~> 4.0'
   s.add_development_dependency 'minitest', '~> 5.4', '>= 5.4.3'
-  s.add_development_dependency 'html-pipeline', '~> 1.0'
+  s.add_development_dependency 'html-pipeline', '~> 2.12'
   s.add_development_dependency 'sanitize', '~> 2.1', '>= 2.1.0'
   s.add_development_dependency 'nokogiri', '~> 1.8.1'
   s.add_development_dependency 'nokogiri-diff', '~> 0.2.0'