Update type of last_played to library.DateType().

Fixes the following exception:

```
** error loading plugin mpdstats
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/beets/plugins.py", line 201, in load_plugins
    namespace = __import__(modname, None, None)
  File "/usr/lib/python2.7/site-packages/beetsplug/mpdstats.py", line 311, in <module>
    class MPDStatsPlugin(plugins.BeetsPlugin):
  File "/usr/lib/python2.7/site-packages/beetsplug/mpdstats.py", line 316, in MPDStatsPlugin
    'last_played': library.Date(),
AttributeError: 'module' object has no attribute 'Date'
```