[Travis] specify better matrix

.travis.yml

-sudo: false
-language: python
-python:
-  - "3.7"
-  - "3.8"
-
 os: linux
 
-before_install:
-  - export BOTO_CONFIG=/dev/null
-
-install:
-  - pip3 install --upgrade pip
-  - pip3 install -r requirements.txt
-  - pip3 freeze
-
-before_script:
-  - psql -c 'create database helios;' -U postgres
-
-script: "python3 -Wall manage.py test"
-
 jobs:
   include:
-  - dist: xenial
+  - python: "3.7"
+    dist: xenial
     addons:
       postgresql: "9.5"
-  - dist: xenial
+  - python: "3.7"
+    dist: xenial
     addons:
       postgresql: "9.6"
-  - dist: xenial
+  - python: "3.7"
+    dist: xenial
     addons:
       postgresql: "10"
-  - dist: bionic
+  - python: "3.8"
+    dist: bionic
     addons:
       postgresql: "9.5"
-  - dist: bionic
+  - python: "3.8"
+    dist: bionic
     addons:
       postgresql: "9.6"
-  - dist: bionic
+  - python: "3.8"
+    dist: bionic
     addons:
       postgresql: "10"
-  - dist: bionic
+  - python: "3.8"
+    dist: bionic
     addons:
       postgresql: "11"
+
+before_install:
+  - export BOTO_CONFIG=/dev/null
+
+install:
+  - pip3 install --upgrade pip
+  - pip3 install -r requirements.txt
+  - pip3 freeze
+
+before_script:
+  - psql -c 'create database helios;' -U postgres
+
+script: "python3 -Wall manage.py test -v 2"