Fix compatibility with dokku 0.4+

commands

@@ -1,11 +1,18 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-source "$PLUGIN_PATH/common/functions"
+
+PLUGIN_BASE_PATH="$PLUGIN_PATH"
+if [[ -n $DOKKU_API_VERSION ]]; then
+  PLUGIN_BASE_PATH="$PLUGIN_ENABLED_PATH"
+fi
+source "$PLUGIN_BASE_PATH/common/functions"
 source "$(dirname "$0")/functions"
 source "$(dirname "$0")/config"
 
-if [[ ! -d $PLUGIN_DATA_ROOT ]]; then
-  dokku_log_fail "$PLUGIN_SERVICE: Please run: sudo dokku plugins-install"
+if [[ $1 == $PLUGIN_COMMAND_PREFIX:* ]]; then
+  if [[ ! -d $PLUGIN_DATA_ROOT ]]; then
+    dokku_log_fail "$PLUGIN_SERVICE: Please run: sudo dokku plugin:install"
+  fi
 fi
 
 case "$1" in