Travis CI: Run Python 2 and Python 3 tests in parallel

.travis.yml

 language: cpp
 cache: ccache
-sudo: required
-dist: trusty
 services:
   - docker
 os:
@@ -9,14 +7,27 @@ os:
 env:
   - JOB=PRE_COMMIT
 
-addons:
-  apt:
-    packages:
-      - git
-      - python
-      - python-pip
-      - python2.7-dev
-  ssh_known_hosts: 13.229.163.131
+matrix:
+  include:
+    - python: 2.7
+      addons:
+        apt:
+          packages:
+            - git
+            - python
+            - python-pip
+            - python2.7-dev
+        ssh_known_hosts: 13.229.163.131
+    - python: 3.7
+      addons:
+        apt:
+          packages:
+            - git
+            - python3.7
+            - python3.7-pip
+            - python3.7-dev
+        ssh_known_hosts: 13.229.163.131
+
 before_install:
   - sudo pip install -U virtualenv pre-commit pip