Updated .gitmodules to select the right branches out of the box

.gitmodules

 [submodule "glow"]
 	path = extern/glow
 	url = https://www.graphics.rwth-aachen.de:9000/Glow/glow.git
-	branch = master
+	branch = develop
 [submodule "glfw"]
 	path = extern/glfw
 	url = https://github.com/glfw/glfw.git
 [submodule "glow-extras"]
 	path = extern/glow-extras
 	url = https://www.graphics.rwth-aachen.de:9000/Glow/glow-extras.git
+	branch = develop
 [submodule "aion"]
 	path = extern/aion
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/aion.git
@@ -23,6 +24,7 @@
 [submodule "polymesh"]
 	path = extern/polymesh
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/polymesh.git
+	branch = develop
 [submodule "extern/assimp"]
 	path = extern/assimp
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/assimp-lean.git
@@ -32,3 +34,4 @@
 [submodule "extern/typed-geometry"]
 	path = extern/typed-geometry
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/typed-geometry.git
+	branch = develop