Bump to mono:2018-08

.gitmodules

@@ -8,7 +8,7 @@
 [submodule "external/mono"]
     path = external/mono
     url = ../../mono/mono.git
-    branch = 2018-04
+    branch = 2018-08
 [submodule "external/opentk"]
     path = external/opentk
     url = ../../mono/opentk.git

Make.config

@@ -62,9 +62,9 @@ XCODE_URL=http://xamarin-storage/bot-provisioning/xcodes/Xcode_9.4.xip
 XCODE_DEVELOPER_ROOT=/Applications/Xcode94.app/Contents/Developer
 
 # Minimum Mono version
-MIN_MONO_VERSION=5.14.0.136
-MAX_MONO_VERSION=5.14.99
-MIN_MONO_URL=https://xamjenkinsartifact.azureedge.net/build-package-osx-mono/2018-04/111/07c8f25fe536dbb7e244f965aa3f9a871f41e953/MonoFramework-MDK-5.14.0.136.macos10.xamarin.universal.pkg
+MIN_MONO_VERSION=5.16.0.15
+MAX_MONO_VERSION=5.16.99
+MIN_MONO_URL=https://xamjenkinsartifact.azureedge.net/build-package-osx-mono/2018-08/7/f123d67e048465fdcc980059e7ec70daf9e0f7cc/MonoFramework-MDK-5.18.0.15.macos10.xamarin.universal.pkg
 
 # Minimum Visual Studio version
 MIN_VISUAL_STUDIO_URL=https://download.visualstudio.microsoft.com/download/pr/11550896/783d2219a348f93b6988fb415951788a/VisualStudioForMac-Preview-7.4.0.985.dmg

external/mono

@@ -1 +1 @@
-Subproject commit d31dbe843a556a5377a66793ee438c6f52695379
+Subproject commit f123d67e048465fdcc980059e7ec70daf9e0f7cc