replace parameter is not present on python2.6

  Include backward compatibility to python2.6

src/zc/buildout/easy_install.py

@@ -705,8 +705,13 @@ class Installer:
                             # recipes. Make sure that whatever correct version
                             # we've just installed is the active version,
                             # hence the ``replace=True``.
-                            ws.add(dist, replace=True)
-
+                            try:
+                              ws.add(dist, replace=True)
+                            except TypeError:
+                              # We are running in python 2.6
+                              # remove the key to force the replacement
+                              ws.by_key.pop(dist.key, None)
+                              ws.add(dist)
             finally:
                 if tmp != self._download_cache:
                     shutil.rmtree(tmp)