Bump to cecil/xamarin-android-d15-3/16d23978 to get scope reading fix (#166)

    commit 16d23978e7084c910b47f84092a5c546d880ae48
    Author: Radek Doulik <[email protected]>
    Date:   Thu Jun 22 21:53:55 2017 +0200

        fix reading scope start

         - in case when the mdb file is out of sync, we might endup with
           start_instruction being null

Author: radekdoulik

.gitmodules

@@ -5,4 +5,4 @@
 [submodule "external/cecil"]
 	path = external/cecil
 	url = https://github.com/mono/cecil.git
-	branch = mono-2017-04
+	branch = xamarin-android-d15-3