[Rivet] Check Python Version in Bootstrap

[Markus Radziej]

Hi,

I discovered, that in Rivet 2.1.2 in the make-plots executable python 
script, in the function try_cmd(args) you attempt to use 
subprocess.check_output(...). This lead to an error while trying to 
generate plots on the lxcluster I am working on. After some debugging, I 
found out that this routine was implemented in python 2.7 and I am 
running python 2.6.

I think it would be best to check for a minimum python version in the 
bootstrap, don't you agree?

Cheers,
Markus Radziej