Add missing phpunit dependency.

Attempt to fix scrutinizer.

.scrutinizer.yml

@@ -1,12 +1,15 @@
 inherit: true
 
-tools:
-  external_code_coverage: false
-
 checks:
-  php:
-    code_rating: true
-    duplication: true
+  php: true
+
+build:
+  nodes:
+    analysis:
+      tests:
+        override: [php-scrutinizer-run]
 
 filter:
-  paths: [src/*, tests/*]
+  paths:
+    - src/*
+    - tests/*

composer.json

@@ -2,7 +2,12 @@
     "name": "bummzack/sortablefile",
     "description": "An extension for SilverStripe 4 that adds sorting to UploadField.",
     "type": "silverstripe-vendormodule",
-    "keywords": ["silverstripe", "CMS", "sortable", "file"],
+    "keywords": [
+        "silverstripe",
+        "CMS",
+        "sortable",
+        "file"
+    ],
     "license": "BSD-3-Clause",
     "authors": [
         {
@@ -12,6 +17,9 @@
     "require": {
         "silverstripe/asset-admin": "1.1.x-dev"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^5.7"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "2.x-dev"