Resolve "Make gitlab run PHPunit tests"

Closes #16

.gitlab-ci.yml

@@ -23,8 +23,8 @@ 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
+  - RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
   - composer install
   script:
   - phpunit tests/*.php