add back bluebird

package.json

@@ -58,7 +58,6 @@
     "request": "^2.75.0"
   },
   "trulyOptionalDependencies": {
-    "bluebird": "^3.5.1",
     "le-acme-core": "^2.1.3",
     "ursa": "^0.9.4"
   },
@@ -66,6 +65,7 @@
     "acme": "^1.0.6",
     "acme-v2": "^1.0.6",
     "asn1js": "^1.2.12",
+    "bluebird": "^3.5.1",
     "certpem": "^1.0.0",
     "le-challenge-fs": "^2.0.2",
     "le-sni-auto": "^2.1.3",