Update composer dependencies and add PHPUnit

Author: kenjis

composer.json

@@ -1,6 +1,6 @@
 {
     "name": "kenjis/codeigniter-ss-twig",
-    "description": "A Simple and Secure Twig integration for CodeIgniter 3.0",
+    "description": "A Simple and Secure Twig integration for CodeIgniter 3.x",
     "keywords": [
         "codeigniter",
         "twig"
@@ -17,7 +17,8 @@
         "twig/twig": "~1.20"
     },
     "require-dev": {
-        "codeigniter/framework": "3.0.*"
+        "codeigniter/framework": "3.1.*",
+        "phpunit/phpunit": "^5.6"
     },
     "extra": {
         "branch-alias": {