Update dependencies

[sjsadowski]

Currently zlib is officially deprecated and other cookbooks with lack of updates listed in the metadata.rb. Probably this should be cleaned up as there's no way to actually use this cookbook right now without doing a workaround.

[chewi]

You've mentioned zlib but it would be helpful to tell us what the others are. I'm not aware of any necessary workarounds. Something happened in the supermarket to cause nginx <3 to pick an ancient version so I've changed this to ~>2.7 in my upcoming pull request. selinux_policy was pinned back due to its dependency on an older yum version but this has just been fixed.