This update for HepMC2 fixes the following issues:

HepMC2 was updated to version 2.06.11:

* HepMC/HepMCDefs.h: Add a HEPMC_HAS_ORDERED_WEIGHTS #define.
* HepMC/WeightContainer.h: Add weights() and weight_names()
public methods.
* src/WeightContainer.cc: Fix print() to use canonical weight
ordering.
* src/GenEventStreamIO.cc: Fix the I/O streaming to preserve the
weight ordering.
* Fixed the tests.