Improve travis config.

.travis.yml

@@ -1,44 +1,22 @@
+dist: bionic
 sudo: false
 language: python
+cache: pip
+install: pip install tox tox-travis
+script: tox -v
+python:
+  - 3.6
+  - 3.7
+  - 3.8
 matrix:
+  fast_finish: true
   include:
-    - python: 3.6
-      dist: xenial
-      env: TOX_ENV=flake8
-    - python: 3.6
-      dist: xenial
-      env: TOX_ENV=py37-django22
-    - python: 3.6
-      dist: xenial
-      env: TOX_ENV=py37-django30
-
-    - python: 3.7
-      dist: xenial
-      env: TOX_ENV=flake8
-    - python: 3.7
-      dist: xenial
-      env: TOX_ENV=py37-django22
-    - python: 3.7
-      dist: xenial
-      env: TOX_ENV=py37-django30
-
     - python: 3.8
-      dist: xenial
       env: TOX_ENV=flake8
-    - python: 3.8
-      dist: xenial
-      env: TOX_ENV=py37-django22
-    - python: 3.8
-      dist: xenial
-      env: TOX_ENV=py37-django30
 addons:
   apt:
     packages:
       - libenchant-dev
-install:
-  - pip install tox
-script:
-  - tox -e $TOX_ENV
 after_success:
   - pip install coveralls
   - coveralls