Updated donate link to point to SO Premium

* Updated tested up to tag.

Resolves #89.

readme.txt

@@ -1,12 +1,12 @@
 === SiteOrigin CSS ===
 Tags: css, design, edit, customize
 Requires at least: 3.9
-Tested up to: 5.1
+Tested up to: 5.2.2
 Stable tag: trunk
 Build time: unbuilt
 License: GPLv2 or later
 Contributors: gpriday
-Donate link: https://siteorigin.com/downloads/contribution/
+Donate link: https://siteorigin.com/downloads/premium/
 
 SiteOrigin CSS is the simple, yet powerful CSS editor for WordPress. It gives you visual controls that let you edit the look and feel of your site in real-time.