Fix CER calculation when BTS is quote

Fixes: bitshares#138

Author: bitphage

bitshares/market.py

@@ -114,7 +114,10 @@ def ticker(self):
         """
         data = {}
         # Core Exchange rate
-        cer = self["quote"]["options"]["core_exchange_rate"]
+        if self["quote"]["id"] == '1.3.0':
+            cer = self["base"]["options"]["core_exchange_rate"]
+        else:
+            cer = self["quote"]["options"]["core_exchange_rate"]
         data["core_exchange_rate"] = Price(
             cer,
             blockchain_instance=self.blockchain