Changing ->timestamps() to use DATETIME.

This has roots in the 2038 problem. MySQL TIMESTAMP columns do not
have the ability to store a date greater than the year 2038. MySQL
DATETIME has no issue on 64-bit platoforms, but it does have the
same issue on 32-bit platforms. Assuming most people will be using
64-bit servers, this change will prevent a huge number of people
running into this issue at all.