.travis.yml: The 'sudo' tag is now deprecated in Travis CI (#154)

[Travis are now recommending removing the __sudo__ tag](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration).

"_If you currently specify __sudo:__ in your __.travis.yml__, we recommend removing that configuration_"

Author: cclauss

.travis.yml

@@ -8,7 +8,6 @@ jobs:
     - python: '3.6'
     - python: '3.7'
       dist: xenial
-      sudo: true
 
 # command to install dependencies
 install: "pip install -r requirements.txt"