Bump to mono/2017-04/1b8c5541 (#651)

jonpryor · web-flow · commit d0b7102b81ab · 2017-06-16T13:15:17.000-04:00
Updates mono's cecil. Context: https://bugzilla.xamarin.com/show_bug.cgi?id=56808
diff --git a/external/mono b/external/mono
@@ -1 +1 @@
-Subproject commit c51d1343f37b5b7a21ee2c0db5188485b7dae295
+Subproject commit 1b8c5541500b458f5bc164b0908d62caaea0648d
