[Developers] /automotive/generation/years_produced should not have an ect of /type/text

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-686

/user/tfmorris noticed that the property /automotive/generation/ 
years_produced has an ect of /type/rawstring, and it should be / 
measurement_unit/time_interval .  Updating the ect of the property  
could break queries - is there anyone that objects to this change, or  
anyone that needs more time to update applications?

Bryan