Use async/await instead of explicit Promise

src/converse-omemo.js

@@ -159,14 +159,13 @@ converse.plugins.add('converse-omemo', {
                     }).then(() => this.buildSessions(devices))
             },
 
-            buildSession (device) {
+            async buildSession (device) {
                 const { _converse } = this.__super__,
                       address = new libsignal.SignalProtocolAddress(device.get('jid'), device.get('id')),
                       sessionBuilder = new libsignal.SessionBuilder(_converse.omemo_store, address),
-                      prekey = device.getRandomPreKey();
+                      prekey = device.getRandomPreKey(),
+                      bundle = await device.getBundle();
 
-                return device.getBundle()
-                    .then(bundle => {
                 return sessionBuilder.processPreKey({
                     'registrationId': parseInt(device.get('id'), 10),
                     'identityKey': u.base64ToArrayBuffer(bundle.identity_key),
@@ -180,7 +179,6 @@ converse.plugins.add('converse-omemo', {
                         'publicKey': u.base64ToArrayBuffer(prekey.key),
                     }
                 });
-                    });
             },
 
             getSession (device) {