Dropping support for old versions of IE

[Sean Farley]

At the request of Mads, I tried to test out my retina pull request on
different versions of IE:

https://bitbucket.org/conservancy/kallithea/pull-request/82/retina-loose-ends

but I couldn't run anything less than IE 11 on my Windows 8 box. Could
we at least drop support for IE < 10?

Alternatively, we could take a stance of "support the latest(ish)
version of IE until someone complains / sends patches."

Thoughts?