Cache composer packages on Appveyor

appveyor.yml

@@ -10,6 +10,7 @@ cache:
 
 init:
   - SET PATH=%PHP_CACHE_DIR%;%PATH%
+  - SET COMPOSER_CACHE_DIR=%PHP_CACHE_DIR%
   - SET COMPOSER_NO_INTERACTION=1
   - SET PHP=0
   - SET ANSICON=121x90 (121x90)