Update Koa documentation (2.15.0)

lib/docs/scrapers/koa.rb

@@ -3,7 +3,7 @@
 module Docs
   class Koa < Github
     self.base_url = 'https://github.com/koajs/koa/tree/master/docs'
-    self.release = '2.14.2'
+    self.release = '2.15.0'
 
     self.root_path = 'api/index.md'
     self.initial_paths = %w[