Update submodule over https instead of ssh in case of no ssh is configured (#14)

diff --git a/.gitmodules b/.gitmodules
index 84361ee..af41f23 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "protocol"]
 	path = protocol
-	url = git@github.com:apache/skywalking-data-collect-protocol.git
+	url = https://github.com/apache/skywalking-data-collect-protocol.git