PHP extension mbstring not listed as dependency in composer.json

[Potherca]

I recently switched over to this package from the PhpSpec version.

When I tried to deploy my application to Heroku I encountered an error:

Call to undefined function Phalcon\Diff\Renderer\Html\mb_strlen() on line 142 in php-diff/src/Diff/Renderer/Html/BaseArray.php

This is caused by the mbstring extension not being registered as a dependency. The workaround for me was to add the dependency in my propjects `composer.json.

    {
        "require" : {
            "ext-mbstring": "*"
        }
    }

As this is trivial to fix, I will send in a pull-request shortly.