Improve install_html_meta_data to handle commented out elib refs

Resolves #552

book/robust.html

@@ -724,12 +724,6 @@ <h1><a href="index.html" style="text-decoration:none;">Underactuated Robotics</a
 PhD thesis, Massachusetts Institute of Technology, September, <span class="year">2014</span>.
 [&nbsp;<a href="http://groups.csail.mit.edu/robotics-center/public_papers/Moore14b.pdf">link</a>&nbsp;]
 
-</li><br>
-<li id=Boyd04a>
-<span class="author">Stephen Boyd and Lieven Vandenberghe</span>, 
-<span class="title">"Convex Optimization"</span>, Cambridge University Press
-, <span class="year">2004</span>.
-
 </li><br>
 <li id=Ebenbauer09>
 <span class="author">Christian Ebenbauer and Tobias Raff and Frank Allgower</span>, 

book/trajopt.html

@@ -1855,12 +1855,6 @@ <h1>The special case of direct shooting without state constraints</h1>
 <span class="publisher">Humanoid Robots (Humanoids), 2017 IEEE-RAS 17th International Conference on</span> , <span class="year">2017</span>.
 [&nbsp;<a href="http://groups.csail.mit.edu/robotics-center/public_papers/Marcucci17.pdf">link</a>&nbsp;]
 
-</li><br>
-<li id=Junge05>
-<span class="author">O. Junge and J. E. Marsden and S. Ober-Bloebaum</span>, 
-<span class="title">"Discrete mechanics and optimal control"</span>, 
-<span class="publisher">Proceedings of the 16th IFAC World Congress</span> , <span class="year">2005</span>.
-
 </li><br>
 <li id=Nganga21>
 <span class="author">John N Nganga and Patrick M Wensing</span>,