.travis.yml: The 'sudo' tag is now deprecated in Travis CI

[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: false__ in your __.travis.yml__, we recommend removing that configuration_"

Also, removed Python 3.4 because it is EOL  https://devguide.python.org/devcycle/#end-of-life-branches

.travis.yml

@@ -1,20 +1,9 @@
 language: python
-
+python:
-matrix:
+  - : 3.5
-  include:
+  - : 3.6
-    - python: 3.4
+  - : 3.7
-      dist: trusty
+dist: xenial
-      sudo: false
-    - python: 3.5
-      dist: trusty
-      sudo: false
-    - python: 3.6
-      dist: trusty
-      sudo: false
-    - python: 3.7
-      dist: xenial
-      sudo: true
-
 services:
  - docker
 install: