Mark minimum phpunit version

composer.json

@@ -30,7 +30,7 @@
         "symfony/process": "~2.1@dev"
     },
     "require-dev": {
-        "phpunit/phpunit": "3.7.*"
+        "phpunit/phpunit": "~3.7.10"
     },
     "suggest": {
         "ext-zip": "Enabling the zip extension allows you to unzip archives, and allows gzip compression of all internet traffic",

composer.lock

@@ -1,5 +1,5 @@
 {
-    "hash": "b7537d80d94e19d0b668b9a88d9f8262",
+    "hash": "38828459a269ef0e409883721853d7fe",
     "packages": [
         {
             "name": "justinrainbow/json-schema",