tinydtls: update to latest version

This change points the submodule to the changed tinydtls repository
which now lives at https://github.com/eclipse/tinydtls and updates
to the latest development HEAD.

Closes PR #531. [1]

[1] https://github.com/obgm/libcoap/pull/531

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "ext/tinydtls"]
 	path = ext/tinydtls
-	url = https://git.eclipse.org/r/tinydtls/org.eclipse.tinydtls
+	url = https://github.com/eclipse/tinydtls.git
     ignore = dirty