Add PHPUnit as a dev dependency

Also has it install, and cache, as part of the before_script for CI.

.gitignore

 .hhconfig
 .project
 .DS_Store
+/vendor/

.gitlab-ci.yml

 before_script:
   - "echo 'assume_php=false' > .hhconfig"
+  - composer install
+
+cache:
+  paths:
+    - vendor/
 
 hack:
   script:

composer.json

@@ -23,5 +23,10 @@
   "support": {
     "issues": "https://git.simon.geek.nz/91-carriage/phpunit-hhi/issues",
     "source": "https://git.simon.geek.nz/91-carriage/phpunit-hhi/tree/master"
-  }
+  },
+  "require-dev": {
+    "phpunit/phpunit": "5.4.*"
+  },
+  "minimum-stability": "dev",
+  "prefer-stable": true
 }