Test against latest 2.1 on travis
Skip primary key in a more reliable way across rails versions

Rails decided to remove Column#primary [1] and this breaks our ability
to detect whether the column is the primary key as we used to, so rely
on the target class' primary key method for that, which should work fine
in all other rails versions.

[1]
https://github.com/rails/rails/commit/05dd3df35db8b2d39ed177f4ccfe112bdfe7726d