log: CallStats log level 'info'

Reduces the log level for the CallStats module to 'info', because recent
changes are adding a lot of debug logs.

logging_config.js

     // Option to disable LogCollector (which stores the logs on CallStats)
     //disableLogCollector: true,
     // Logging level adjustments for verbose modules:
-    'modules/xmpp/strophe.util.js': 'log'
+    'modules/xmpp/strophe.util.js': 'log',
+    'modules/statistics/CallStats.js': 'info'
 };