Fix getUnread query

Booleans are not valid keys in indexeddb.
https://www.w3.org/TR/IndexedDB/#dfn-valid-key

// FREEBIE

js/background.js

@@ -176,7 +176,7 @@
             received_at    : now,
             conversationId : source,
             type           : 'incoming',
-            unread         : true
+            unread         : 1
         });
 
         return message;

js/models/conversations.js

@@ -56,9 +56,9 @@
             return unreadMessages.fetch({
                 index: {
                     // 'unread' index
-                    name: 'unread',
-                    lower: [conversationId, true],
-                    upper: [conversationId, true]
+                    name  : 'unread',
+                    lower : [conversationId],
+                    upper : [conversationId, Number.MAX_VALUE],
                 }
             }).always(function() {
                 resolve(unreadMessages);