Remove invalid zc.buildout>=2.7.1+slapos001 requirement

Local version labels (+<local>) are no longer supported in unequality
version requirements syntax, e.g. `project == x.y.z+local` is ok but
`project >= x.y.z+local` is not ok.

setup.py

@@ -43,7 +43,7 @@ setup(
     ],
   install_requires=[
     'setuptools',
-    'zc.buildout >=2.7.1+slapos001, <2.7.2',
+    'zc.buildout',
     ],
   extras_require=extras_require,
   tests_require=extras_require['test'],