Bump to mono/2017-04/c51d1343 (#644)

jonpryor · web-flow · commit 89828ede5ed9 · 2017-06-12T19:03:40.000-04:00
Context: https://bugzilla.xamarin.com/show_bug.cgi?id=57222
diff --git a/external/mono b/external/mono
@@ -1 +1 @@
-Subproject commit 4498dc4a62fe7dcd4e3b29898144f41d6396f577
+Subproject commit c51d1343f37b5b7a21ee2c0db5188485b7dae295
