Externals Update, 21.2 branch (2019.02.04.)

Updated all projects to atlasexternals-1.0.48. This is to fix problems picking up some external sources from AFS. (atlasexternals@1.0.47...1.0.48) See ATLINFR-2911 and ATLINFR-2891.

Also, I updated AthDerivation to once again use the same version of the externals as everybody else, since https://sft.its.cern.ch/jira/browse/SPI-1208 should be in a good shape now. Still, before merging this in, we'll need to make sure that this latter ticket is fully resolved first.