title: Travis-ci installs packages from requirements.txt file automatically
body: <p>I have Django application based on docker-compose file.
  Somehow travis autoinstalls packages from requirements.txt in project repo and its failing my build cause I don't have gcc package. 
  I want to run all actions (tests, linters) in docker container not directly in project repo. </p>
  
  <p>Here is my travis-ci.yml file:</p>
  
  <pre><code>---
  dist: xenial
  services:
    - docker
  language: python
  python:
    - "3.7"
  script:
    - docker compose up --build
    - docker exec web flake8
    - docker exec web mypy my_project
    - docker exec web safety check -r requirements.txt
    - docker exec web python -m pytest --cov my_project -vvv -s
  </code></pre>
  
  <p>And begin of travis log:</p>
  
  <pre><code>$ git checkout -qf bab09dee57a707a5cd0a353d6f50bb66fd90a095
  0.01s$ source ~/virtualenv/python3.7/bin/activate
  $ python --version
  Python 3.7.1
  $ pip --version
  pip 19.0.3 from /home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pip (python 3.7)
  $ pip install -r requirements.txt
  ...
  py_GeoIP.c:23:19: fatal error: GeoIP.h: No such file or directory
  compilation terminated.
  error: command 'gcc' failed with exit status 1
  ...
  </code></pre>
  
  <p>Do you have any idea why travis behaves like this?</p>