Updating go depencancies. Switching to go1.6 vendoring (#2949)

Makefile

@@ -1,5 +1,8 @@
 .PHONY: build package run stop run-client run-server stop-client stop-server restart-server restart-client start-docker clean-dist clean nuke check-style check-unit-tests test dist setup-mac prepare-enteprise run-client-tests setup-run-client-tests cleanup-run-client-tests test-client build-linux build-osx build-windows
 
+# For golang 1.5.x compatibility (remove when we don't want to support it anymore)
+GO15VENDOREXPERIMENT=1
+
 # Build Flags
 BUILD_NUMBER ?= $(BUILD_NUMBER:)
 BUILD_DATE = $(shell date -u)