svg images in ReST documents omitted due to use of <object> tag filtering

Example ReST:

```
.. image:: https://scan.coverity.com/projects/621/badge.svg
   :target: https://scan.coverity.com/projects/621
   :alt: Coverity Scan Build Status
```

Live example: https://github.com/ged-lab/khmer/blob/d7a079c8c22062e06687ad0bacabe93f35230210/README.rst
