`date_time` is no longer valid

[igbanam]

Hi everyone,

I was going through the docs trying to get my first Amber app off the ground. On trying to do a migration, I realized the suggested date_time alongside datetime are not valid options for DB fields in Amber. I used timestamp and this worked.

I think the docs should be updated to reflect this.

09:29 $ amber -v
Amber CLI (amberframework.org) - v0.9.0

Crystal 0.26.1 (2018-08-27)

LLVM: 6.0.1
Default target: x86_64-apple-macosx