add spdy as optional

package.json

@@ -11,9 +11,12 @@
     "greenlock": "^2.2.8",
     "le-challenge-fs": "^2.0.8",
     "le-sni-auto": "^2.1.4",
-    "le-store-certbot": "^2.0.5",
+    "le-store-certbot": "^2.1.0",
     "redirect-https": "^1.1.5"
   },
+  "optionalDependencies": {
+    "spdy": "^3.4.7"
+  },
   "devDependencies": {
     "express": "^4.16.3",
     "express-basic-auth": "^1.1.5",