Allow custom cache dir

We are running oh-my-zsh on a shared install and the only issue is each
user having a custom cache dir. This will allow us to set the cache dir
to ZSH_CACHE_DIR=~/oh-my-zsh/cache/ .

lib/completion.zsh

@@ -38,7 +38,7 @@ cdpath=(.)
 
 # Use caching so that commands like apt and dpkg complete are useable
 zstyle ':completion::complete:*' use-cache 1
-zstyle ':completion::complete:*' cache-path $ZSH/cache/
+zstyle ':completion::complete:*' cache-path $ZSH_CACHE_DIR
 
 # Don't complete uninteresting users
 zstyle ':completion:*:*:*:users' ignored-patterns \