Fix template deprecation warnings generated in puppet 3

In puppet 3 you'll see deprecations warnings like:

```
Warning: Variable access via 'gem_path' is deprecated. Use '@gem_path' instead. template[/.../modules/passenger/templates/passenger-conf.erb]:1
   (at /.../puppet/modules/passenger/templates/passenger-conf.erb:1:in `result')
Warning: Variable access via 'gem_path' is deprecated. Use '@gem_path' instead. template[/.../puppet/modules/passenger/templates/passenger-conf.erb]:2
   (at /.../puppet/modules/passenger/templates/passenger-conf.erb:2:in `result')
Warning: Variable access via 'passenger_ruby' is deprecated. Use '@passenger_ruby' instead. template[/.../puppet/modules/passenger/templates/passenger-conf.erb]:3
   (at /.../puppet/modules/passenger/templates/passenger-conf.erb:3:in `result')
```