updating eos to 4.2.20-20180424100234git959ab4f with improved WFE timings

continuousintegration/docker/ctafrontend/cc7/etc/yum/pluginconf.d/versionlock.list

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+0:eos-srm-4.2.20-20180424100234git959ab4f.el7.cern.x86_64
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+0:eos-testkeytab-4.2.20-20180424100234git959ab4f.el7.cern.x86_64
 1:python2-xrootd-4.8.2-1.el7.*
 1:python3-xrootd-4.8.2-1.el7.*
 1:xrootd-4.8.2-1.el7.*