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/3a3f9c4eb9219f52e3203f38ac38de87b31daeb4 [autoroller] remove unused diff field. (iannucci@chromium.org)

TBR=iannucci@chromium.org

Change-Id: I0dd92aec95984574b577785f51d141fbf6b50606
Recipe-Tryjob-Bypass-Reason: Autoroller
Bugdroid-Send-Email: False
Reviewed-on: https://chromium-review.googlesource.com/476313
Reviewed-by: <recipe-roller@chromium.org>
Commit-Queue: <recipe-roller@chromium.org>

infra/config/recipes.cfg

@@ -3,7 +3,7 @@
   "deps": {
     "recipe_engine": {
       "branch": "master",
-      "revision": "a7afc5a62ed0e2c5237785128159a65921816796",
+      "revision": "3a3f9c4eb9219f52e3203f38ac38de87b31daeb4",
       "url": "https://chromium.googlesource.com/external/github.com/luci/recipes-py.git"
     }
   },