fix six package path.

--strip_prefix doesn't seem to work.

WORKSPACE

@@ -10,7 +10,6 @@ new_http_archive(
   url = "https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz#md5=34eed507548117b2ab523ab14b2f8b55",
   sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
   build_file = "six.BUILD",
-  strip_prefix = "six-1.10.0",
 )
 
 bind(

six.BUILD

@@ -1,5 +1,12 @@
+genrule(
+  name = "copy_six",
+  srcs = ["six-1.10.0/six.py"],
+  outs = ["six.py"],
+  cmd = "cp $< $(@)",
+)
+
 py_library(
   name = "six",
-  srcs = ["six-1.10.0/six.py"],
+  srcs = ["six.py"],
   visibility = ["//visibility:public"],
 )