Remove API key from ‘handler ready’ message

The API key is potentially sensitive if logs are handled through a
third-party service.

Fixes #283

lib/bugsnag.rb

@@ -47,7 +47,7 @@ def configure(config_hash=nil)
       @logged_ready = false unless defined?(@logged_ready)
 
       if configuration.api_key && !@logged_ready
-        log "Bugsnag exception handler #{VERSION} ready, api_key=#{configuration.api_key}"
+        log "Bugsnag exception handler #{VERSION} ready"
         @logged_ready = true
       end
     end