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

.travis.yml

 language: cpp
 cache: ccache
-sudo: required
-dist: trusty
 services:
   - docker
 os:
@@ -9,7 +7,10 @@ os:
 env:
   - JOB=PRE_COMMIT
 
-addons:
+matrix:
+  include:
+    - python: 2.7
+      addons:
         apt:
           packages:
             - git
@@ -17,6 +18,16 @@ addons:
             - 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