Add GitLab CI

• Different set of boxes, there isn't a 7.2 in the current series and doesn't seem like that'll changes
• Correct PHPUnit version for PHP version, as Travis handles

Fixes #2 (closed)

.gitlab-ci.yml

+variables:
+  # Configure mysql service (https://hub.docker.com/_/mysql/)
+  MYSQL_DATABASE: wordpress_tests
+  MYSQL_ROOT_PASSWORD: mysql
+
+cache:
+  paths:
+    - $HOME/.composer
+
+before_script:
+  # Install dependencies
+
+  # update the docker
+  - apt-get clean
+  - apt-get -yqq update
+
+  # install the required packages for the running CI tests
+  - |
+    apt-get -yqqf install lsb-release zip unzip subversion mysql-client libicu-dev libpng-dev libbz2-dev --fix-missing
+
+    if [[ $(lsb_release -a) =~ "stretch" ]]; then
+      apt-get -yqqf install default-libmysqlclient-dev --fix-missing
+    else
+      apt-get -yqqf install libmysqlclient-dev --fix-missing
+    fi
+
+  # PHP extensions
+  - docker-php-ext-install mbstring mysqli intl gd zip bz2
+  - docker-php-ext-enable mbstring mysqli intl gd zip bz2
+
+  # Install Composer
+  - curl -sS https://getcomposer.org/installer | php
+  - mv composer.phar /usr/local/bin/composer
+  - export PATH="$HOME/.composer/vendor/bin:$PATH"
+
+  # Set up WordPress tests
+  - bash bin/install-wp-tests.sh $MYSQL_DATABASE root $MYSQL_ROOT_PASSWORD mysql latest true
+
+  # PHPUnit
+  - |
+    if [[ $(php -v) =~ "PHP 7." ]]; then
+      composer global require "phpunit/phpunit=6.1.*"
+    else
+      composer global require "phpunit/phpunit=4.8.*"
+    fi
+
+  # Install PHPCS and WPCS
+  - composer global require automattic/vipwpcs
+  - phpcs --config-set installed_paths $HOME/.composer/vendor/wp-coding-standards/wpcs,$HOME/.composer/vendor/automattic/vipwpcs
+
+PHPunit:PHP5.3:MySQL:
+  image: tetraweb/php:5.3
+  services:
+    - mysql:5.6
+  script:
+    - phpcs -n
+    - phpunit
+  allow_failure: true
+
+PHPunit:PHP5.6:MySQL:
+  image: php:5.6-stretch
+  services:
+    - mysql:5.6
+  script:
+    - phpcs -n
+    - phpunit
+
+PHPunit:PHP7.0:MySQL:
+  image: php:7.0-stretch
+  services:
+    - mysql:5.6
+  script:
+    - phpcs -n
+    - phpunit
+
+PHPunit:PHP7.1:MySQL:
+  image: php:7.1-stretch
+  services:
+    - mysql:5.6
+  script:
+    - phpcs -n
+    - phpunit
+
+PHPunit:PHP7.2:MySQL:
+  image: php:7.2-stretch
+  services:
+    - mysql:5.6
+  script:
+    - phpcs -n
+    - phpunit