software/erp5: backport a fix for pyrsistent

This solves the error occuring installing python2 softwares:

    error: find_namespace: directive is unsupported on Python < 3.3
    An error occurred when trying to install pyrsistent 0.14.5. Look above this message for any errors that were output by easy_install.
    While:
      Updating slapos-cookbook-dependencies.
      Getting distribution for 'pyrsistent==0.14.5'.

software/erp5/software.cfg

 [buildout]
 extends =
   ../../stack/erp5/buildout.cfg
+  
+
+# backports fixes
+# pyrsistent 0.14.5 has a setup_requires, which is installed regardless of
+# buildout versions and pickup a version incompatible with python 2,
+# preventing installation.
+# pyrsistent 0.16.1 does not have this setup_requires, which makes the
+# software installable again.
+[versions]
+pyrsistent = 0.16.1