Use Go 1.7.3 for psiphond

Server/Dockerfile-binary-builder

@@ -1,6 +1,6 @@
 FROM alpine:latest
 
-ENV GOLANG_VERSION 1.7.1
+ENV GOLANG_VERSION 1.7.3
 ENV GOLANG_SRC_URL https://golang.org/dl/go$GOLANG_VERSION.src.tar.gz
 
 RUN set -ex \