Bump mono requirement to fix mmp tests. (#1066)

This fixes the following mmptests:

1) Classic_NewRefCount_Warns (Xamarin.MMP.Tests.MMPTests.Classic_NewRefCount_Warns)
2) SystemMono_SmokeTest (Xamarin.MMP.Tests.MMPTests.SystemMono_SmokeTest)

That fails like this with earlier versions of Mono 4.8:

    Undefined symbols for architecture i386:
      "_mono_btls_x509_lookup_method_mono_init", referenced from:
         -u command line option
      "_mono_btls_x509_name_list_add", referenced from:
         -u command line option
      "_mono_btls_x509_name_list_free", referenced from:
         -u command line option
      "_mono_btls_x509_name_list_get_count", referenced from:
         -u command line option
      "_mono_btls_x509_name_list_get_item", referenced from:
         -u command line option
      "_mono_btls_x509_name_list_new", referenced from:
         -u command line option
    ld: symbol(s) not found for architecture i386
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

    error MM5109: Native linking failed with error code 1.  Check build log for details.

Make.config

@@ -53,9 +53,9 @@ XCODE_URL=http://xamarin-storage/storage/bot-provisioning/Xcode_8.1_GM_Seed.xip
 XCODE_DEVELOPER_ROOT=/Applications/Xcode81-GM.app/Contents/Developer
 
 # Minimum Mono version
-MIN_MONO_VERSION=4.4.0.148
+MIN_MONO_VERSION=4.8.0.269
 MAX_MONO_VERSION=4.9.99
-MIN_MONO_URL=http://download.mono-project.com/archive/4.4.0/macos-10-universal/MonoFramework-MDK-4.4.0.148.macos10.xamarin.universal.pkg
+MIN_MONO_URL=http://bosstoragemirror.blob.core.windows.net/wrench/mono-4.8.0/e5/e51aa0a7043a8185e264d6332ac7bc1f8f87cd39/MonoFramework-MDK-4.8.0.269.macos10.xamarin.universal.pkg
 
 # Minimum Xamarin Studio version
 MIN_XAMARIN_STUDIO_URL=https://files.xamarin.com/~rolf/XamarinStudio-6.1.0.4373.dmg