Bump min mono version for XM system apps.

Using a system mono < 5.18 results in a TypeLoadException:

    Could not resolve type with token 01000032 from typeref (expected class 'Mono.ISystemDependencyProvider' in assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089')

Make.config

@@ -52,8 +52,8 @@ MAX_MONO_VERSION=5.18.99
 MIN_MONO_URL=https://xamjenkinsartifact.azureedge.net/build-package-osx-mono/2018-08/96/f21a48dad5fefde8b987a96b45ac9c4e42e35754/MonoFramework-MDK-5.18.0.124.macos10.xamarin.universal.pkg
 
 # Minimum Mono version for Xamarin.Mac apps using the system mono
-MIN_XM_MONO_VERSION=5.14.0.136
-MIN_XM_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_XM_MONO_VERSION=5.18.0.24
+MIN_XM_MONO_URL=https://xamjenkinsartifact.azureedge.net/build-package-osx-mono/2018-08/96/f21a48dad5fefde8b987a96b45ac9c4e42e35754/MonoFramework-MDK-5.18.0.124.macos10.xamarin.universal.pkg
 
 # Minimum Visual Studio version
 MIN_VISUAL_STUDIO_URL=https://bosstoragemirror.azureedge.net/vsmac/7a/7aff2dc1f28d711d11d63d79b2a4c49cda217189/VisualStudioForMac-Preview-7.7.0.1470.dmg