Pass :type => :bigint to t.belongs_to to retain 64 bit columns.

When we merged [1] via [2]:

[1] https://github.com/ManageIQ/manageiq/pull/895/
[2] https://github.com/ManageIQ/rails/pull/10

We inherited new behavior: we now are required to specify the :type => :bigint when specifying t.belongs_to and t.references whereas these were always bigint using our prior version of our rails fork.

The t.references was fixed in:
https://github.com/ManageIQ/manageiq/pull/895

This commit fixes t.belongs_to.