Check coding style

.travis.yml

@@ -18,6 +18,14 @@ test-extensions-job-template: &test-extensions-job-template
 
 jobs:
   include:
+    - stage: Pre-test checks
+      name: Check coding style
+      if: type = pull_request
+      language: go
+      go: '1.13'
+      before_script: GO111MODULE=on go install mvdan.cc/sh/v3/cmd/shfmt
+      script: shfmt -d -s -ln posix -i 0 -ci -kp install-php-extensions scripts/common scripts/travisci-test-extensions scripts/travisci-update-readme scripts/update-readme
+
     - <<: *test-extensions-job-template
       name: Test extensions on Alpine 3.7
       env: DOCKER_DISTRO=alpine3.7