error: running homebrew as root is extremely dangerous and no longer supported. as homebrew does not drop privileges on installation you would be giving all build scripts full access to your system.
I was able to solve the problem by using chown on the folder:
sudo chown -R "$USER":admin /usr/local
Also you'll (most probably) have to do the same on /Library/Caches/Homebrew:
sudo chown -R "$USER":admin /Library/Caches/Homebrew