Merge pull request #46 from cclauss/patch-2

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

.travis.yml

-language: cpp
+language: python
+compiler: gcc
 cache: ccache
-dist: trusty
-services:
-  - docker
-os:
-  - linux
-env:
-  - JOB=PRE_COMMIT
-
 addons:
   apt:
     packages:
       - git
-      - python
-      - python-pip
-      - python2.7-dev
-  ssh_known_hosts: 13.229.163.131
+  # ssh_known_hosts: 13.229.163.131
+env:
+  - JOB=PRE_COMMIT
+matrix:
+  include:
+    - python: 2.7
+    - python: 3.7
+      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+
 before_install:
-  - sudo pip install -U virtualenv pre-commit pip
+  - pip install --upgrade pip
+  - pip install pre-commit
 
 script:
   - exit_code=0