dark magic to revive travis

It seems that some process inside travis is using port no 9000. Hence
adding this line in the travis.yml to change common_site_config.json and
use port no 9001 for socket_io

.travis.yml

@@ -33,6 +33,7 @@ before_script:
   - bench reinstall --yes
   - bench build
   - bench scheduler disable
+  - sed -i 's/9000/9001/g' sites/common_site_config.json
   - bench start &
   - sleep 10