Update .gitmodules with public repo urls

Seems like the right thing to do, plus readthedocs only supports public urls.

.gitmodules

 [submodule "sims/external/gem5"]
 	path = sims/external/gem5
-	url = git@github.com:simbricks/gem5.git
+	url = https://github.com/simbricks/gem5.git
 [submodule "sims/external/qemu"]
 	path = sims/external/qemu
-	url = git@github.com:simbricks/qemu.git
+	url = https://github.com/simbricks/qemu.git
 [submodule "sims/external/ns-3"]
 	path = sims/external/ns-3
-	url = git@github.com:simbricks/ns-3.git
+	url = https://github.com/simbricks/ns-3.git
 [submodule "sims/external/femu"]
 	path = sims/external/femu
-	url = git@github.com:simbricks/femu.git
+	url = https://github.com/simbricks/femu.git