Roll back yaml version in DEPS to 2.1.11

Rejecting duplicate keys breaks download of existing Dart packages.
BUG=https://github.com/dart-lang/pub/issues/1479
R=lrn@google.com
CC=mit@google.com,nweiz@google.com,kustermann@google.com

Review URL: https://codereview.chromium.org/2548313002 .
diff --git a/DEPS b/DEPS
index 44e124f..4f5d541 100644
--- a/DEPS
+++ b/DEPS
@@ -120,7 +120,7 @@
   "WebCore_rev": "@a86fe28efadcfc781f836037a80f27e22a5dad17",
   "when_tag": "@0.2.0+2",
   "which_tag": "@0.1.3+1",
-  "yaml_tag": "@2.1.12",
+  "yaml_tag": "@2.1.11",
   "zlib_rev": "@c3d0a6190f2f8c924a05ab6cc97b8f975bddd33f",
 }