Add getAccountSid() method to UsageTrigger

This was missing for some reason.

Author: Ameya Lokare

src/main/java/com/twilio/sdk/resource/instance/UsageTrigger.java

@@ -220,6 +220,15 @@ public String getUri() {
         return getProperty("Uri");
     }
 
+    /**
+     * Get the account sid
+     *
+     * @return account sid
+     */
+    public String getAccountSid() {
+        return getProperty("AccountSid");
+    }
+
     /**
      * Delete
      *