Use HTTPS rather than SSH for submodules so Travis can run them.

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "src/examples/DT2"]
     path = src/examples/DT2
-    url = git@github.com:cvan/DT2.git
+    url = git://github.com/cvan/DT2.git
 [submodule "src/examples/FullScreenMario"]
 	path = src/examples/FullScreenMario
-	url = git@github.com:cvan/FullScreenMario.git
+	url = git://github.com/cvan/FullScreenMario.git