Track submodule heads.

`git submodule update --remote` will now sync the submodules
to their master branch.

Change-Id: I192ee8153d9538a599ce63fd5d51ade9741bc6fd
Reviewed-on: http://gerrit.dmdirc.com/3597
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

.gitmodules

 [submodule "modules/parser"]
 	path = modules/parser
 	url = git://dmdirc.com/parser
+	branch = master
 [submodule "modules/plugins"]
 	path = modules/plugins
 	url = git://dmdirc.com/plugins
+	branch = master
 [submodule "modules/defaults"]
 	path = modules/defaults
 	url = git://dmdirc.com/defaults
+	branch = master
 [submodule "modules/util"]
 	path = modules/util
 	url = git://dmdirc.com/util
+	branch = master
 [submodule "modules/installer"]
 	path = modules/installer
 	url = git://dmdirc.com/installer
+	branch = master