v2.1.7: make bluebird truly optional

package.json

@@ -1,10 +1,10 @@
 {
   "name": "le-sni-auto",
-  "version": "2.1.6",
+  "version": "2.1.7",
   "description": "An auto-sni strategy for registering and renewing letsencrypt certificates using SNICallback",
   "homepage": "https://git.coolaj86.com/coolaj86/le-sni-auto.js",
   "main": "index.js",
-  "dependencies": {
+  "trulyOptionalDependencies": {
     "bluebird": "^3.5.1"
   },
   "devDependencies": {},
@@ -13,7 +13,7 @@
   },
   "repository": {
     "type": "git",
-    "url": "git+https://git.coolaj86.com/coolaj86/le-sni-auto.js.git"
+    "url": "https://git.coolaj86.com/coolaj86/le-sni-auto.js.git"
   },
   "keywords": [
     "le-sni",