Added description to composer.json

When executing "composer validate", an error is generated because there
is no description. Also removed a couple of tabs and replaced them with
spaces to neaten it up slightly.
diff --git a/composer.json b/composer.json
index e21aaed..44c54b4 100644
--- a/composer.json
+++ b/composer.json
@@ -1,9 +1,10 @@
 {
+    "description" : "Dependencies for CodeIgniter's testing environment",
     "name" : "ellislab/codeigniter",
     "require": {
         "php": ">=5.2.4"
     },
     "require-dev": {
-      "mikey179/vfsStream": "*"
-		}
-}
\ No newline at end of file
+        "mikey179/vfsStream": "*"
+    }
+}