Updates the used strophejs version in dependencies (1.2.15).

package-lock.json

       "dev": true
     },
     "strophe.js": {
-      "version": "github:jitsi/strophejs#1501521d305e405f4dea3f51a5473f504672a7b3"
+      "version": "1.2.15",
+      "resolved": "https://registry.npmjs.org/strophe.js/-/strophe.js-1.2.15.tgz",
+      "integrity": "sha512-aM5SCLltSLKubPNil28ieJ03I+15jcVX02c1/7SBVIUWRfwfxwondRJSMJpB7OBss5b3jCNxpTqig8nXncJ5yg=="
     },
     "strophejs-plugin-disco": {
       "version": "0.0.2",

package.json

     "sdp-interop": "0.1.12",
     "sdp-simulcast": "0.2.1",
     "sdp-transform": "2.3.0",
-    "strophe.js": "github:jitsi/strophejs#1.2.14-1",
+    "strophe.js": "1.2.15",
     "strophejs-plugin-disco": "0.0.2",
     "webrtc-adapter": "github:webrtc/adapter#1eec19782b4058d186341263e7d049cea3e3290a",
     "yaeti": "1.0.1"