Switch loopy remote repository to gitlab.tiker.net

It usually takes 3 days to mirror to github...

.gitmodules

 [submodule "python/loopy"]
 	path = python/loopy
-	url = https://github.com/inducer/loopy.git
+	url = https://gitlab.tiker.net/inducer/loopy.git
 [submodule "python/ufl"]
 	path = python/ufl
 	url = https://bitbucket.org/dokempf/ufl.git