Fix cache directory upgrading - do not try to remove the destination directory.
Composer supports four types.
Merge pull request #2405 from discordier/fix-upgrade-cache

Fix cache directory upgrading - do not try to (re)move the active cache dir.
Merge pull request #2406 from websafe/patch-1

Composer supports four types.
Add stronger warning in docs about #<ref> hack, refs #2410