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.

_auto_deps.py

@@ -8,7 +8,9 @@ install_requires=[
                   # incorrect AES results.
                   "pycryptopp >= 0.5",
                   "zfec >= 1.1.0",
-                  "simplejson >= 1.4",
+
+                  # We had a unicode problem with simplejson 1.8.1 on dapper -- see ticket #543.
+                  "simplejson > 1.8.1",
 
                   "zope.interface",
                   "Twisted >= 2.4.0",

setup.py

@@ -208,10 +208,6 @@ if 'darcsver' in sys.argv[1:]:
 if not os.path.exists('PKG-INFO'):
     setup_requires.append('setuptools_darcs >= 1.1.0')
 
-# Previous versions seems to have unicode issues
-# See Bug #543
-setup_requires.append('simplejson >= 2.0.5')
-
 class ShowSupportLib(Command):
     user_options = []
     def initialize_options(self):