Serve jQuery from cookie-free Google APIs

jQuery.com uses Google Analytics and so sets a cookie when you visit
the site. When you request jquery.js from its CDN, code.jquery.com,
you take a hit from sending your previous GA cookie in the request.

Google Hosted Libraries[1] and googleapis.com never set cookies.

[1] https://developers.google.com/speed/libraries/devguide

_includes/footer.html

 <!-- Bootstrap core JavaScript
 ================================================== -->
 <!-- Placed at the end of the document so the pages load faster -->
-<script src="https://code.jquery.com/jquery-1.10.2.min.js"></script>
+<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
 <script src="{{ page.base_url }}dist/js/bootstrap.js"></script>
 
 <script src="http://platform.twitter.com/widgets.js"></script>