I had the same issue.
The problem was that I replaced fabpot/php-cs-fixer with friendsofphp/php-cs-fixer without replacing it in composer.lock as well. I ran
composer self-update
composer clear-cache
However, each time I tried to execute "composer update" it hung at updating dependencies. You can see this by running
composer update -vvv
So, at last, I rolled back the changes made to require-dev in composer.json and all went well updating Magento version to 2.1.10
Then, I decided to
composer remove fabpot/php-cs-fixer
And then
composer require --dev friendsofphp/php-cs-fixer
However, installation fails because the symfony/console version requirements of friendsofphp/php-cs-fixer v.2.8 require PHP 7.1. That's the problem, because, Magento 2.1 does not support this version of php. Therefore, friendsofphp/php-cs-fixer can only be installed with Magento 2.2