Updated default.html.twig for twitter Bootstrap compatibility

According to twitter Bootstrap documentation, the toggable tags markup
need to have the data-toggle="tab" attribute in the <a> element and
not in its <li> parent.

See http://twitter.github.io/bootstrap/javascript.html#tabs

The data-toggle attribute has been moved accordingly.
Merge pull request #49 from alch/bootstrap-compat

Updated default.html.twig for twitter Bootstrap compatibility