Routes are ran one by one from top to bottom. The higher a route is the higher precedence it has.

A / route will match / on any domain (by default routes don't care about domain, only the path) therefore if you place your / route at the top it will match on crm.laravel.dev/ and not go any further. Place your subdomain specific route at the top:

// IF the domain is crm.laravel.dev
Route::group(array('domain' => 'crm.laravel.dev'), function() 
{ 
    // THEN match a GET request to /
    Route::get('/', function() 
    { 
        return 'Hello from my subdomain!'; 
    }); 
});

// match a GET request to / on ANY domain
Route::get('/', function() 
{ 
    return 'Hello from my main site!'; 
});