edits .gitmodules, fixes libbson url

was getting socket error timeouts on `git submodule update --init` within `autogen.sh`, this change makes it work as intended.

Author: rubicks

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "src/libbson"]
 	path = src/libbson
-	url = git://github.com/mongodb/libbson.git
+	url = https://github.com/mongodb/libbson.git