Update @sls-next/lambda-at-edge to v1.5.1

Update to the latest version of @sls-next/lambda-at-edge. This update
has no impact on next-on-netlify.

Changelog: https://github.com/serverless-nextjs/serverless-next.js/blob/master/packages/lambda-at-edge/CHANGELOG.md

Author: FinnWoelm

package-lock.json

@@ -44,7 +44,7 @@
     "wait-on": "^5.0.1"
   },
   "dependencies": {
-    "@sls-next/lambda-at-edge": "1.2.0-alpha.3",
+    "@sls-next/lambda-at-edge": "^1.5.1",
     "fs-extra": "^9.0.0",
     "next-aws-lambda": "^2.4.1"
   }