Update libuv sumodule URL to use the git:// protocol.

Numerous people seem to have trouble with the https:// protocol.
If you want to use a different protocol, just edit the git config
file (.git/config) and update the setting under the deps/libuv
stanza:

    [submodule "deps/libuv"]
    	url = git://github.com/JuliaLang/libuv.git

Put whatever URL you want to use in there.

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "deps/libuv"]
 	path = deps/libuv
-	url = https://github.com/JuliaLang/libuv.git
+	url = git://github.com/JuliaLang/libuv.git