Composer: how can I install another dependency without updating old ones?

I have a project with a few dependencies and I’d like to install another one, but I’d like to keep the others the way they are. So I’ve edited the composer.json, but if I run composer install, I get the following output:

Installing dependencies from lock file
Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.
Your requirements could not be resolved to an installable set of packages.

Problem 1
    - laravel/framework dev-master requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
    - laravel/framework dev-master requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
    - Installation request for laravel/framework dev-master -> satisfiable by laravel/framework dev-master.

First of all, I do have mcrypt installed, so I don’t know why it’s complaining about that there.

So, how can I install this new dependency?

My composer.json:

{
    "require": {

        "opauth/opauth": "*",
        "opauth/facebook": "*",
        "opauth/google": "*",
        "opauth/twitter": "*",

        "imagine/Imagine": "dev-develop",

        "laravel/framework": "4.*",
        "loic-sharma/profiler": "dev-master"
    },
    "autoload": {
        "classmap": [
            "app/libraries",
            "app/commands",
            "app/controllers",
            "app/models",
            "app/database/migrations",
            "app/tests/TestCase.php"
        ]
    },
    "minimum-stability": "dev"
}