GELFHandler is deprecated by graypy

Hi,

There have been changes in graypy package. This is breaking flask-graylog throwing `AttributeError: module 'graypy' has no attribute 'GELFHandler'`

`GELFHandler has been replaced with GELFUDPHandler` & `GELFTcpHandler being restructured into GELFTCPHandler`

https://github.com/severb/graypy/issues/105#issuecomment-474859565