Include install functions in upgrade() method

Doing this in init() does not work, as the plugin is never initialized
in an install/upgrade context.

Fixes #55

Author: dregad

Snippets.php

@@ -54,10 +54,6 @@ public function hooks() {
 
 	public function init() {
 		require_once("Snippets.API.php");
-
-		if( plugin_needs_upgrade( $this ) ) {
-			require_once( dirname( __FILE__ ) . '/install_functions.php' );
-		}
 	}
 
 	/**
@@ -175,6 +171,13 @@ public function schema() {
 		);
 	}
 
+	public function upgrade( $p_schema ) {
+		if( $p_schema == 1 ) {
+			require_once( dirname( __FILE__ ) . '/install_functions.php' );
+		}
+		return true;
+	}
+
 	/**
 	 * RESTful route for Snippets Pattern Help (tooltip).
 	 *