circleci: Update the used Docker image to one with OMNeT++ 6.0pre9

v3.6.8 for OMNeT++ 6 pre 9 compatibility

Statistics: Fixed warmup period for compound source expressions, see …

…bug inet-framework#549.

prod: fixes for NED errors (added missing emulation showcase feature)

Minor fixes for Windows build

general: fix to build with OMNeT++ 5.5

generic: fix to build with both OMNeT++ 5.3 (or later, including 6.0pre)