Use expand_path to locate JFace JARs

When using Warbler and bundling the swt gem into a JAR,
JRuby was unable to find the JFace JARs, which were collected
using File.dirname. JRuby *could* find the SwtBot
extensions, which were collected using File.expand_path. This commit
simply uses the File.expand_path strategy for both scenarios.

lib/swt/full.rb

@@ -99,7 +99,7 @@ def self.bot
 end
 
 module JFace
-  Dir[File.dirname(__FILE__) + "/../../vendor/jface/*.jar"].each do |jar_fn|
+  Dir[File.expand_path "../../../vendor/jface/*.jar", __FILE__].each do |jar_fn|
     require jar_fn
   end