add php-gd and use php7 image

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 # Select image from https://hub.docker.com/_/php/
-image: php:5.6
+image: php:7
 
 # Select what we should cache
 cache:
@@ -10,6 +10,7 @@ before_script:
 # Install git, the php image doesn't have installed
 - apt-get update -yqq
 - apt-get install git -yqq
+- apt-get install php-gd -yqq
 
 # Install mysql driver
 - docker-php-ext-install pdo_mysql
@@ -33,10 +34,4 @@ test:mysql:
   script:
   - vendor/bin/phpunit --configuration phpunit_mysql.xml --coverage-text
 
-# We test PHP7 with MySQL, but we allow it to fail
-test:php7:mysql:
-  image: php:7
-  script:
-  - vendor/bin/phpunit --configuration phpunit_mysql.xml --coverage-text
-  allow_failure: true