Fix duplicate key error on masternode sync

-Removes the unique constraint on the 'addr' field from the masternode collection

NOTE: If you are experiencing a mongo duplicate key error while running a masternode sync, the easiest way to correct the problem is as follows:
1) #Stop the explorer
2) git pull
3) mongo
4) use explorerdb
5) db.masternodes.drop()
6) exit
7) #Restart the explorer
8) /path/to/explorer/scripts/sync.sh /path/to/node masternodes

models/masternode.js

@@ -7,7 +7,7 @@ var MasternodeSchema = new Schema({
   txhash: { type: String, default: "" },
   outidx : { type: Number, default: 0},
   status : { type: String, default: "" },
-  addr: { type: String, unique: true, index: true },
+  addr: { type: String, index: true },
   version : { type: Number, default: 0},
   lastseen: { type: Number, default: 0 },
   activetime: { type: Number, default: 0 },