Adds strophe.js and plugin-disco as a dependency to fix build problem.

Bumps lib-jitsi-meet to latest. There was a problem that jitsi-meet build fail if anybody touches package.json (including PR testing), this happen after start using custom strophe.js from github:jitsi/strophejs.
The error:
    ERROR in ../strophejs-plugin-disco/lib/strophe.disco.js
    Module not found: Error: Can't resolve 'strophe.js' in '/Users/dminkov/dev/jitsi-meet/node_modules/strophejs-plugin-disco/lib'
     @ ../strophejs-plugin-disco/lib/strophe.disco.js 4:126-147 4:196-227
     @ ./modules/xmpp/xmpp.js
     @ ./JitsiConnection.js
     @ ./JitsiMeetJS.js
     @ ./index.js

Without strophejs-plugin-disco jitsi-meet builds but on runtime loading fail with:
Error: Missing strophe-plugins (disco plugin is required)!

FIXME: We should remove this once strophe.js releases new version and we are back to the official one inside lib-jitsi-meet.

package-lock.json

       }
     },
     "lib-jitsi-meet": {
-      "version": "github:jitsi/lib-jitsi-meet#4ac0af12e26a3012dc59eca06eb823b1a0602826",
+      "version": "github:jitsi/lib-jitsi-meet#f9644b5c7093d61690e36a008aa7fc96a793234d",
       "requires": {
         "async": "0.9.0",
         "current-executing-script": "0.1.3",
         "strophejs-plugin-disco": "0.0.2",
         "webrtc-adapter": "github:webrtc/adapter#1eec19782b4058d186341263e7d049cea3e3290a",
         "yaeti": "1.0.1"
-      },
-      "dependencies": {
-        "strophe.js": {
-          "version": "github:jitsi/strophejs#1501521d305e405f4dea3f51a5473f504672a7b3"
-        }
       }
     },
     "load-json-file": {

package.json

     "jquery-i18next": "1.2.0",
     "js-md5": "0.6.1",
     "jwt-decode": "2.2.0",
-    "lib-jitsi-meet": "github:jitsi/lib-jitsi-meet#4ac0af12e26a3012dc59eca06eb823b1a0602826",
+    "lib-jitsi-meet": "github:jitsi/lib-jitsi-meet#f9644b5c7093d61690e36a008aa7fc96a793234d ",
     "lodash": "4.17.4",
     "moment": "2.19.4",
     "nuclear-js": "1.4.0",
     "redux": "3.7.2",
     "redux-thunk": "2.2.0",
     "strophe.js": "github:jitsi/strophejs#1.2.14-1",
+    "strophejs-plugin-disco": "0.0.2",
     "styled-components": "1.3.0",
     "url-polyfill": "github:github/url-polyfill#39734186de44612bc5a16eb25f5407adcc5b2e7c",
     "uuid": "3.1.0",