Bump watch-mono to latest mono master (#645)

Get in the fix for https://github.com/mono/mono/pull/3429 which should make the corlib step on the macios-mac-master lane green again on Wrench.
2016-08-23 16:41:58 +02:00 · 2016-08-23 16:41:58 +02:00 · de581556b8
--- a/external/watch-mono
+++ b/external/watch-mono
@ -1 +1 @@
-Subproject commit b97fad34be3b34168f3345b2b80f0bf2a79b1be4
+Subproject commit 208650e6da9ff8ffa3a4ee7a591c4e4fa5b48b62