proc_open() has been disabled for security reasons

Then your hosting provider has disabled those options. If you have access to php.ini you can enable them yourself

Edit the php.ini file, and find the line starting with:

disable_functions =.......

Remove all functions from that line that you want to be able to use, save the file and restart your webserver (apache / nginx)/

If you cannot edit php.ini, you're probably out of luck with this provider (you could ask them to turn the functions on in your environment, but they probably wont)

As Lostdreamer said, it's a server setting. If you have control, you can change it. If not don't fret.

If you get the error message while using Composer, it shouldn't matter. It'll just find another way to do what it needs to do.