centos and fedora dep install does system update

[zacharywhitley]

The install_pkgs.sh script, for centos and fedora, does a full system upgrade without confirmation by running yum -y upgrade.

I can understand the desire to have the latest code but it might be a bit much when you're just expecting it to install the required dependencies. I found myself seeing a ton of rpm package upgrades and saying, "whooooooh no way. I'm not prepared for that right now"

I would suggest removing it and only including required packages.

[zacharywhitley]

I was going to put in a separate PR but I accidentally included it in the PR https://github.com/zeutro/openabe/pull/30