setup: move the requirement on simplejson from setup.py to _auto_deps.py, and loosen...

setup: move the requirement on simplejson from setup.py to _auto_deps.py, and loosen it from >= 2.0.5 to > 1.8.1
We'll see if this fixes the tests on all of our current buildslaves, and if it does then I'll be happy to leave it at "> 1.8.1" for now, even though I don't know exactly what versions of simplejson changed exactly what behavior that interacts with exactly what environment.  See http://allmydata.org/trac/tahoe/ticket/534 for uncertainties.