Bump opower to 0.15.7 (home-assistant#154351)

Author: tronikos

homeassistant/components/opower/manifest.json

@@ -8,5 +8,5 @@
   "iot_class": "cloud_polling",
   "loggers": ["opower"],
   "quality_scale": "bronze",
-  "requirements": ["opower==0.15.6"]
+  "requirements": ["opower==0.15.7"]
 }