Update vcpkg port files before installing packages

Current AppVeyor image includes a very old vcpkg snapshot. Because of
that, this doesn't include https://github.com/Microsoft/vcpkg/pull/5258
specifically
https://github.com/Microsoft/vcpkg/pull/5258/commits/c895bb658d52bee471169460c711fdd90e3d5b7b

Because of that, find_package only works for find_package(nlohmann-json)
but not find_package(nlohmann_json). Updating the ports cache should fix
that.

appveyor.yml

@@ -16,6 +16,7 @@ install:
     - set PATH=%PATH%;C:\mingw-w64\x86_64-7.2.0-posix-seh-rt_v5-rev1\mingw64\bin
     - mingw32-make.exe --version
     - call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
+    - cd "C:\Tools\vcpkg"&& git pull && .\bootstrap-vcpkg.bat && cd %APPVEYOR_BUILD_FOLDER%
     - vcpkg install
             nlohmann-json:%PLATFORM%-windows
             boost-asio:%PLATFORM%-windows