Revamped/simplify Travis configuration

* Drop the "sudo" for Python 3.7 job,
* Using the "magic" matrix utility to make it more readable,
* Upgraded the VM to Bionic

.travis.yml

@@ -1,25 +1,12 @@
-sudo: false
-dist: xenial
+dist: bionic
 language: python
-install:
-    - pip install tox
-
-script:
-    - tox -e $TOXENV
-
+cache: pip
+install: pip install tox tox-travis
+script: tox -v
+python:
+- 2.7
+- 3.5
+- 3.6
+- 3.7
 matrix:
   fast_finish: true
-  include:
-    # Python version is just for the look on travis.
-    - python: 2.7
-      env: TOXENV=py27
-
-    - python: 3.5
-      env: TOXENV=py35
-
-    - python: 3.6
-      env: TOXENV=py36
-
-    - python: 3.7
-      env: TOXENV=py37
-      sudo: true

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## master (unreleased)
 
-Nothing here yet.
+* Revamped/Simplified Travis configuration.
 
 ## 1.0.0 (2020-05-05)