Do not depend on slapos.libnetworkcache, which is optional

Since slapos.libnetworkcache needs configuration, the user already
pulls it explicitely. Do not annoy those who don't use it.

In our case, it breaks offline builds with Python 2.6 when the
download-cache is prepared with Python 2.7: argparse missing.

setup.py

@@ -24,7 +24,6 @@ setup(name=name,
       install_requires=[
         'setuptools', # namespaces
         'zc.buildout', # plays with buildout
-        'slapos.libnetworkcache>=0.13.1', # Uses helper new in this version
         ],
       extras_require={
         'test' : ['zope.testing'],