Update protoc-gen-validate to latest HEAD (3204975) (#497)

This update brings in
https://github.com/lyft/protoc-gen-validate/pull/59 which will allow
envoy/data-plane-api to build using a newer version of rules_go.
Necessary for https://github.com/envoyproxy/envoy/issues/2578

Signed-off-by: Snow Pettersen <snowp@squareup.com>

bazel/repositories.bzl

@@ -3,7 +3,7 @@ GOGOPROTO_SHA = "342cbe0a04158f6dcb03ca0079991a51a4248c02" # Oct 7, 2017
 PROMETHEUS_SHA = "6f3806018612930941127f2a7c6c453ba2c527d2" # Nov 02, 2017
 OPENCENSUS_SHA = "993c711ba22a5f08c1d4de58a3c07466995ed962" # Dec 13, 2017
 
-PGV_GIT_SHA = "d08da4114b44ba22f904906f8608c7593925ff0f"
+PGV_GIT_SHA = "3204975f8145b7d187081b7034060012ae838d17"
 
 def api_dependencies():
     native.git_repository(