Test in IE ≤ 7

[mathiasbynens]

…so the README can be updated.

[mgol]

I wanted to test after trying Safari 4.0.5 (#5) but test pages don't load for me at all in all IEs, including 11 and even Edge 12.10240.

Works fine on other browsers (Chrome 45, Firefox 41, Safari 4.0.5 & 9.0).