End-tags don't have to match case to terminate a tag.

Summary:
<b>xxx</B> ends the bold tag just fine.  More importantly for us,
<script>...</SCRIPT> ends a script tag just fine.

Test Plan: tests/i18nize_templates_test.py

Reviewers: jlfwong

Reviewed By: jlfwong

Differential Revision: http://phabricator.khanacademy.org/D14594