Remove duplication on composer.json and update phpunit version (#4)

Author: p16

composer.json

@@ -9,7 +9,7 @@
 		"validation"
 	],
 	"require-dev": {
-		"phpunit/phpunit": "4.1.*"
+		"phpunit/phpunit": "5.1.*"
 	},
 	"license": "MIT",
 	"authors": [
@@ -23,12 +23,9 @@
 		"issues": "https://github.com/Ekman/Luhn-Algorithm/issues",
 		"source": "https://github.com/Ekman/Luhn-Algorithm"
 	},
-	"require-dev": {
-		"phpunit/phpunit": "3.7.*"
-	},
 	"autoload": {
 		"psr-4": {
 			"Nekman\\LuhnAlgorithm\\": "src"
 		}
 	}
-}
+}