Remove dependencies needed to support Node.js <= 0.12

Author: marco-c

package.json

@@ -29,12 +29,7 @@
   },
   "homepage": "https://github.com/marco-c/web-push#readme",
   "dependencies": {
-    "array.prototype.find": "^2.0.0",
     "asn1.js": "^4.8.1",
-    "bluebird": "^3.3.5",
-    "buffer-compare-shim": "^1.0.0",
-    "buffer-equals-polyfill": "^1.0.0",
-    "create-ecdh": "^4.0.0",
     "http_ece": "^0.5.1",
     "jws": "^3.1.3",
     "minimist": "^1.2.0",