Download and cache dotNetFx40_Client_setup.exe in Appveyor

appveyor.yml

@@ -1,10 +1,17 @@
 version: 1.1.0.{build}
 install:
-- cinst nsis
+- cmd: cinst nsis
+- ps: |
+    $url='http://download.microsoft.com/download/7/B/6/7B629E05-399A-4A92-B5BC-484C74B5124B/dotNetFx40_Client_setup.exe'
+    if (!(Test-Path (Split-Path $url -Leaf))) {
+        Start-FileDownload $url
+    }
 assembly_info:
-  patch: true
+    patch: true
 configuration:
 - Debug
 - Release
 before_build:
 - nuget restore
+cache:
+- dotNetFx40_Client_setup.exe