Appveyor: use "choco install" rather than "cinst".

Chocolatey 2.0 got rid of "cinst" and some other wrappers.  It was a
shortcut for "choco install" that was deprecated in 1.0 (but none of the
AppVeyor Windows VMs had 1.x pre-installed, so we didn't see a
deprecation warning that might have indicated that we should be using
"choco install") and whose replacement doesn't appear to be documented
anywhere on the Chocolatey site.

.appveyor.yml

@@ -10,7 +10,7 @@ matrix:
   fast_finish: true
 
 install:
-  - cinst winflexbison
+  - choco install winflexbison
   - win_flex --version
   - win_bison --version
   - appveyor DownloadFile https://www.winpcap.org/install/bin/WpdPack_4_1_2.zip