Update timber-helper.php

My Timber-generated pages were failing HTML validation due to some improperly closed standalone tags. This changes those tags to the correct formatting. I left the close slash for backward compatibility with XHTML. There may be other tags, but these were the ones that quickly came to mind.
Merge branch 'patch-1' of github.com:discern/timber into discern-patch-1

* 'patch-1' of github.com:discern/timber:
  Update timber-helper.php
making test for discern's patch, and then a patch on-top based on test
Merge branch 'discern-patch-1'

* discern-patch-1:
  making test for discern's patch, and then a patch on-top based on test
  Update timber-helper.php