Merge pull request #981 from dstufft/no-pre

Don't set PIP_PRE to 1 during install

Author: dstufft

virtualenv.py

@@ -891,7 +891,6 @@ def space_path2url(p):
         "PIP_FIND_LINKS": findlinks,
         "PIP_USE_WHEEL": "1",
         "PIP_ONLY_BINARY": ":all:",
-        "PIP_PRE": "1",
         "PIP_USER": "0",
     }