Drop Docker

.travis.yml

-language: cpp
+language: python
+compiler: gcc
 cache: ccache
-services:
-  - docker
-os:
-  - linux
+addons:
+  apt:
+    packages:
+      - git
+  # ssh_known_hosts: 13.229.163.131
 env:
   - JOB=PRE_COMMIT
-
 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
       dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
-      addons:
-        apt:
-          sources:
-            - deadsnakes
-          packages:
-            - git
-            - python3.7-venv
-        ssh_known_hosts: 13.229.163.131
 
 before_install:
-  - python --version
-  - sudo pip install -U virtualenv pre-commit pip
+  - pip install --upgrade pip
+  - pip install pre-commit
 
 script:
   - exit_code=0