Update minimum WP requirement

See bradyvercher#43.

Author: GaryJones

README.md

@@ -3,7 +3,7 @@
 A WordPress plugin to easily embed Gists via oEmbed or shortcode.
 
 __Contributors:__ [Brady Vercher](https://github.com/bradyvercher), [Gary Jones](https://github.com/GaryJones)  
-__Requires:__ May work on WordPress versions as far back as 2.9.0.  
+__Requires:__ 3.5.0  
 __Tested up to:__ 3.9  
 __License:__ [GPL-2.0+](http://www.gnu.org/licenses/gpl-2.0.html)