Temporarily fixing the code style failure in CI

Looks like the shallow clone only contains the master branch. Should it contain the target branch instead?

.travis.yml

@@ -306,6 +306,11 @@ jobs:
  if: type = pull_request
  php: 7.2
  script:
+ - |
+ if [ $TRAVIS_BRANCH != "master" ]; then
+ git fetch origin $TRAVIS_BRANCH;
+ git branch origin/$TRAVIS_BRANCH FETCH_HEAD
+ fi
  - git merge-base origin/$TRAVIS_BRANCH $TRAVIS_PULL_REQUEST_SHA || git fetch origin +refs/pull/$TRAVIS_PULL_REQUEST/merge --unshallow
  - wget https://github.com/diff-sniffer/git/releases/download/0.1.0/git-phpcs.phar
  - php git-phpcs.phar origin/$TRAVIS_BRANCH...$TRAVIS_PULL_REQUEST_SHA