Resolve "Make gitlab run PHPunit tests"

Closes #16

.gitlab-ci.yml

@@ -23,10 +23,14 @@ unit_tests:
   image: "php:7.4"
   before_script:
   - apt-get update -yqq
-  - apt-get install -y git php-common
-  - apt-get install -y composer
+  - apt-get install -y git zip
+  - curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
   - composer install
   script:
-  - phpunit tests/*.php
+  - ./vendor/bin/phpunit tests/*.php
   tags:
   - spes
+  cache:
+    key: $CI_COMMIT_REF_SLUG
+    paths:
+    - "vendor/"