Hide the verbose 7z extraction output on AppVeyor.

Author: tritao

appveyor.yml

@@ -37,7 +37,7 @@ environment:
 
 install:
   - appveyor DownloadFile https://dl.dropboxusercontent.com/u/194502/CppSharp/llvm_windows_x86.7z
-  - 7z x llvm_windows_x86.7z -o%DEPS_PATH%\llvm -y
+  - 7z x llvm_windows_x86.7z -o%DEPS_PATH%\llvm -y > nul
 
 build_script:
   - build\premake5.exe --file=build\premake4.lua %VS_VERSION%