STORM-2855: Revert to 2017Q4 Ubuntu image in Travis to fix build
STORM-2855: Revert to 2017Q4 Ubuntu image in Travis to fix build

srdo/storm,pczb/storm,hmcc/storm,srdo/storm,0x726d77/storm,0x726d77/storm,erikdw/storm,hmcl/storm-apache,srishtyagrawal/storm,kishorvpatil/incubator-storm,0x726d77/storm,0x726d77/storm,kishorvpatil/incubator-storm,kishorvpatil/incubator-storm,srdo/storm,hmcl/storm-apache,hmcl/storm-apache,kishorvpatil/incubator-storm,0x726d77/storm,erikdw/storm,hmcc/storm,srishtyagrawal/storm,erikdw/storm,hmcl/storm-apache,pczb/storm,erikdw/storm,srdo/storm,pczb/storm,kishorvpatil/incubator-storm,kishorvpatil/incubator-storm,hmcc/storm,0x726d77/storm,pczb/storm,hmcl/storm-apache,srishtyagrawal/storm,hmcc/storm,srishtyagrawal/storm,pczb/storm,srishtyagrawal/storm,srdo/storm,hmcc/storm,pczb/storm,hmcl/storm-apache,0x726d77/storm,srishtyagrawal/storm,erikdw/storm,hmcc/storm,srdo/storm,srishtyagrawal/storm,hmcc/storm,erikdw/storm,erikdw/storm,hmcl/storm-apache,pczb/storm,kishorvpatil/incubator-storm,srdo/storm