submod: Switch lnprototest to clone from github.com/rustyrussell/lnprototest

This is the official root repository, and some tools (pip for example) doesn't like
pointing to repos that don't actually contain the commit that we point to. Funny
that Github actually shows non-existent commits in clones (bitcoin/bitcoin has been
impersonated before too).

Author: cdecker

.gitmodules

@@ -16,7 +16,7 @@
 	url = https://github.com/valyala/gheap
 [submodule "external/lnprototest"]
 	path = external/lnprototest
-	url = https://github.com/niftynei/lnprototest.git
+	url = https://github.com/rustyrussell/lnprototest.git
 	branch = nifty/ripemd160-fallback
 [submodule "external/lowdown"]
 	path = external/lowdown