CI: added GitLab CI

.gitlab-ci.yml

+stages:
+- tests
+
+tests-php7.1:
+  stage: tests
+  tags: ["grifart/php7.1-with-gulp-and-all-php-modules"]
+  image: grifart/php7.1-with-gulp-and-all-php-modules
+  
+  before_script:
+    - composer install --ansi --no-interaction
+  script:
+    - vendor\bin\tester -c tests\php.ini tests