[Rivet] All hail King David, Squisher of Antique Rivet Memory Bugs

[Andy Buckley]

Hi Riveters,

Please pause for a moment in what you're doing and say a little prayer 
to David G, who has just fixed a bug in Rivet's internal memory handling 
that defeated me back in 2010 and has been (innocuously!) lurking among 
us for more than 5 years.

The replacement of boost::shared_ptr with std::shared_ptr on the trunk 
finally provoked a sufficiently strong response to track it down. And 
now I can sleep at night.

Thanks David, I owe you a drink ;-)
Andy

PS. In other news, the trunk is filling up with good things. Rivet 2.5.0 
is going to be a significant one.

-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow