Update logo reference

Author: wooorm

readme.md

@@ -1,4 +1,4 @@
-# ![NLCST](https://cdn.rawgit.com/wooorm/nlcst/master/logo.svg)
+# ![NLCST][logo]
 
 Natural language for human and machine.
 
@@ -159,6 +159,8 @@ MIT © Titus Wormer
 
 <!--Definitions-->
 
+[logo]: https://cdn.rawgit.com/wooorm/nlcst/f677854/logo.svg
+
 [unist]: https://github.com/wooorm/unist
 
 [retext]: https://github.com/wooorm/retext