[SECURITY] Fedora 26 Update: varnish-5.1.3-4.fc26

This is Varnish Cache, a high-performance HTTP accelerator. Varnish Cache stores web pages in memory so web servers don=EF=BF=BD=EF=BF =BD=EF=BF=BDt have to create the same web page over and over again. Varnish Cache serves pages much faster than any application server; giving the website a significant speed up. Documentation wiki and additional information about Varnish Cache is available on: https://www.varnish-cache.org/