Roll recipe dependencies (trivial).

This is an automated CL created by the recipe roller. This CL rolls recipe
changes from upstream projects (e.g. depot_tools) into downstream projects
(e.g. tools/build).

More info is at https://goo.gl/zkKdpD. Use https://goo.gl/noib3a to file a bug
(or complain)

recipe_engine:
  https://crrev.com/a239f1551fb3ba35d72b9ad4d0cf80ee08c29b22 Vendor virtualenv, update to 15.1.0. (dnj@chromium.org)

TBR=martiniss@chromium.org,phajdan.jr@chromium.org

Recipe-Tryjob-Bypass-Reason: Autoroller
Bugdroid-Send-Email: False
Review-Url: https://codereview.chromium.org/2512373002

infra/config/recipes.cfg

@@ -5,5 +5,5 @@ deps {
   project_id: "recipe_engine"
   url: "https://chromium.googlesource.com/external/github.com/luci/recipes-py.git"
   branch: "master"
-  revision: "a1c2338808c1a2a02d520f4078d21f766b43df5b"
+  revision: "a239f1551fb3ba35d72b9ad4d0cf80ee08c29b22"
 }