playground: add package from golang.org/x/talks

This adds the package golang.org/x/talks/2016/applicative/google so
Sameer's slide linked below will work correctly.

https://talks.golang.org/2016/applicative.slide#34

Change-Id: I14254792babfe08a57719121377eaa525e78a340
Reviewed-on: https://go-review.googlesource.com/24320
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/sandbox/Dockerfile b/sandbox/Dockerfile
index 4fc39c5..2173f2c 100644
--- a/sandbox/Dockerfile
+++ b/sandbox/Dockerfile
@@ -28,7 +28,8 @@
 	golang.org/x/tour/pic \
 	golang.org/x/tour/reader \
 	golang.org/x/tour/tree \
-	golang.org/x/tour/wc
+	golang.org/x/tour/wc \
+	golang.org/x/talks/2016/applicative/google
 
 # add tour packages under their old import paths (so old snippets still work)
 RUN mkdir -p $GOPATH/src/code.google.com/p/go-tour && \