Fix handling of sync message with blocked numbers

[scottnonnenberg]

Previously we were crashing whenever we got a sync message with blocked messages.

[liliakai]

Alternatively, could we drop the envelope arg from the caller since it's not used in handleBlocked?

[scottnonnenberg]

I think it's worth keeping, for consistency in handleSyncMessage:

        } else if (syncMessage.contacts) {
            return this.handleContacts(envelope, syncMessage.contacts);
        } else if (syncMessage.groups) {
            return this.handleGroups(envelope, syncMessage.groups);
        } else if (syncMessage.blocked) {
            return this.handleBlocked(envelope, syncMessage.blocked);
        } else if (syncMessage.request) {
            console.log('Got SyncMessage Request');
            return this.removeFromCache(envelope);
        } else if (syncMessage.read && syncMessage.read.length) {
            console.log('read messages',
                    'from', envelope.source + '.' + envelope.sourceDevice);
            return this.handleRead(envelope, syncMessage.read);
        } else if (syncMessage.verified) {
            return this.handleVerified(envelope, syncMessage.verified);
        } else {