Resolve "Make gitlab run PHPunit tests"

Closes #16

.gitlab-ci.yml

@@ -28,6 +28,10 @@ unit_tests:
   - composer install
   script:
   - find -ipath "*phpunit*"
-  - phpunit tests/*.php
+  - ./vendor/bin/phpunit tests/*.php
   tags:
   - spes
+  cache:
+    key: $CI_COMMIT_REF_SLUG
+    paths:
+    - "vendor/"