vendor: update vendor upspin.io to 2e37c3b, use new dep prune rules

Change-Id: Ibc77a8ae3df26458de8c8d83cd3e7356cff03c89
Reviewed-on: https://upspin-review.googlesource.com/18760
Reviewed-by: Rob Pike <r@golang.org>

Gopkg.toml

@@ -16,3 +16,9 @@ ignored = ["golang.org/x/text/collate"]
 [[constraint]]
   branch = "master"
   name = "upspin.io"
+
+[prune]
+  non-go = true
+  unused-packages = true
+  go-tests = true
+

vendor-update.sh

@@ -5,17 +5,6 @@
 
 # The dep command can be obtained with "go get github.com/golang/dep/cmd/dep".
 
-# Update the upspin.io package.
 dep ensure -update upspin.io
-# Remove any vendored packages we don't use.
-dep prune
-# Delete test files.
-find vendor -name '*_test.go' -delete
-# Delete Google Cloud JSON API schemas.
-find vendor -name '*-api.json' -delete
-# Delete the cacheserver sources, which appear in the vendor directory because
-# we depend on cacheutil (a bug or deficiency in dep).
-find vendor/upspin.io/cmd/cacheserver -type f -depth 1 -delete
-
 git add vendor Gopkg.lock
 git gofmt