Partial Externals Downgrade, 21.2 branch (2019.01.31.)
Downgraded AthDerivation back to atlasexternals-1.0.45. This is to avoid a problem with the Cairo package that showed up in the nightly. (See https://sft.its.cern.ch/jira/browse/SPI-1208.)
This undoes !20751 (merged) just for the AthDerivation project. Since I really don't want to downgrade the externals for the analysis releases...
Tagging @egramsta and @boeriu. We should push this in before the start of the nightly even if the CI doesn't finish...