Rename a function

Avoid confusing this operation with actual receipts, which are something
else.

// FREEBIE

js/models/conversations.js

@@ -219,7 +219,7 @@
                     };
                 });
                 if (read.length > 0) {
-                    textsecure.messaging.sendReadReceipts(read);
+                    textsecure.messaging.syncReadMessages(read);
                 }
             });
         }