Use special name "." for the branches in .gitmodules

It is used to indicate that the submodule branch should be the same as
the branch of the parent repository - devel for devel, master for master.

It's a new feature of git, appearing in version 2.10, but most users
should not really care. It only affects the behavior of "git submodule
update --remote" when updating the submodules.

.gitmodules

@@ -1,10 +1,11 @@
 [submodule "librfxcodec"]
 	path = librfxcodec
 	url = https://github.com/neutrinolabs/librfxcodec.git
+	branch = .
 	ignore = untracked
 
 [submodule "libpainter"]
 	path = libpainter
 	url = https://github.com/neutrinolabs/libpainter.git
-	branch = devel
+	branch = .
 	ignore = untracked