Set DOTENV_LOOK_UPWARDS=1 by default

Change `_dotenv_hash_pair`, adding a version suffix to trigger
re-authentication.

Fixes https://github.com/Tarrasch/zsh-autoenv/issues/3

autoenv.zsh

@@ -25,8 +25,7 @@ _dotenv_stack_entered=()
 
 _dotenv_hash_pair() {
   local env_file=$1
-  env_shasum=$(shasum $env_file | cut -d' ' -f1)
-  echo "$env_file:$env_shasum"
+  echo "$env_file:$env_shasum:1"
 }
 
 _dotenv_authorized_env_file() {