v2.6.1: update to acme-v2 with better challenges

package.json

@@ -1,6 +1,6 @@
 {
   "name": "greenlock",
-  "version": "2.6.0",
+  "version": "2.6.1",
   "description": "Let's Encrypt for node.js on npm",
   "main": "index.js",
   "files": [
@@ -39,7 +39,7 @@
   },
   "dependencies": {
     "acme": "^1.2.0",
-    "acme-v2": "^1.3.1",
+    "acme-v2": "^1.5.0",
     "cert-info": "^1.5.0",
     "le-challenge-fs": "^2.0.2",
     "le-sni-auto": "^2.1.3",