Update dependency koa to v2.15.2

Author: renovate[bot]

package.json

@@ -15,7 +15,7 @@
     "graphql-tag": "2.9.2",
     "graphql-tools": "3.0.5",
     "is_js": "0.9.0",
-    "koa": "2.5.2",
+    "koa": "2.15.2",
     "koa-bodyparser": "4.2.1",
     "koa-morgan": "1.0.1",
     "koa-passport": "4.1.0",