Fixes failing test for JRuby

in JRuby FileUtils do not change
current dir when moving files

```ruby
p Dir.pwd
run_generator [app_root]
p Dir.pwd
FileUtils.mv(app_root, app_moved_root, :verbose => true)
p Dir.pwd
```

```
"/Users/arunagw/checkouts/rails/railties"
"/Users/arunagw/checkouts/rails/railties/test/fixtures/tmp/myapp"
mv /Users/arunagw/checkouts/rails/railties/test/fixtures/tmp/myapp /Users/arunagw/checkouts/rails/railties/test/fixtures/tmp/myapp_moved
"/Users/arunagw/checkouts/rails/railties/test/fixtures/tmp/myapp"
```

We need to cd in new Dir.