Update Travis configuration
Update Travis configuration

 - Switch from Trusty to Xenial (we only support Xenial and Bionic)
 - Switch from Python 3.6 to Python 3.5 (we support it)
 - Remove commands that do not work for a long time

REGOVAR/Regovar,REGOVAR/Regovar,REGOVAR/Regovar,REGOVAR/Regovar