Skip builds on Travis CI with "[skip travis]"

.travis.yml

sudo: false

language: php

git:
  depth: 5

php:
  - 7.0

before_install:
  - if echo "$TRAVIS_COMMIT_MESSAGE" | grep -F -q "[skip travis]" ; then echo "[skip travis] has been found, exiting" && exit 0 ; else echo "[skip travis] has not been found, continuing" ; fi

script:
  - echo "OK, I should not see this"

Update: use $TRAVIS_COMMIT_MESSAGE instead of git show -s HEAD

Source: travis-ci/travis-build#936

This doesn't work for skipping pull requests that have [skip travis] in the commit message.