Remove ext-mbstring dependency.

mbstring is not a hard dependency for 2.x. The Multibyte class can shim
up the missing extension.

Refs cakephp#2564

Author: markstory

composer.json

@@ -19,8 +19,7 @@
     },
     "require": {
         "php": ">=5.2.8",
-        "ext-mcrypt": "*",
-        "ext-mbstring": "*"
+        "ext-mcrypt": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "3.7.*",