Fix TravisCI build file to update framework to netcoreapp2.1
Fix TravisCI build file to update framework to netcoreapp2.1

mdissel/Marten,mysticmind/marten,mysticmind/marten,ericgreenmix/marten,mdissel/Marten,JasperFx/Marten,mysticmind/marten,ericgreenmix/marten,mysticmind/marten,JasperFx/Marten,ericgreenmix/marten,JasperFx/Marten,ericgreenmix/marten