Remove "update = checkout" option for submodules

This reverts af16d8b. It caused
problems with older git versions (e.g. 1.8.5) that do not recognize the
checkout option: warning: unknown update mode 'checkout' suggested for
submodule '3rdparty/catch' Skipping submodule '3rdparty/catch'

Also specify the branch for the catch submodule, to be in line with the
other submodules.

.gitmodules

@@ -1,29 +1,24 @@
 [submodule "3rdparty/catch"]
 	path = 3rdparty/catch
 	url = https://github.com/wxWidgets/Catch.git
-	update = checkout
+	branch = wx
 [submodule "src/zlib"]
 	path = src/zlib
 	url = https://github.com/wxWidgets/zlib.git
 	branch = wx
-	update = checkout
 [submodule "src/png"]
 	path = src/png
 	url = https://github.com/wxWidgets/libpng.git
 	branch = wx
-	update = checkout
 [submodule "src/expat"]
 	path = src/expat
 	url = https://github.com/wxWidgets/libexpat.git
 	branch = wx
-	update = checkout
 [submodule "src/tiff"]
 	path = src/tiff
 	url = https://github.com/wxWidgets/libtiff.git
 	branch = wx
-	update = checkout
 [submodule "src/jpeg"]
 	path = src/jpeg
 	url = https://github.com/wxWidgets/libjpeg-turbo.git
 	branch = wx
-	update = checkout