Log Message:
Update ruby-activerecord31 to 3.1.4.

Tweak COMMENT.

## Rails 3.1.4 (unreleased) ##

*   Fix a custom primary key regression *GH 3987*

    *Jon Leighton*

*   Perf fix (second try): don't load records for `has many :dependent =>
    :delete_all` *GH 3672*

    *Jon Leighton*

*   Fix accessing `proxy_association` method from an association extension
    where the calls are chained. *GH #3890*

    (E.g. `post.comments.where(bla).my_proxy_method`)

    *Jon Leighton*

*   Perf fix: MySQL primary key lookup was still slow for very large
    tables. *GH 3678*

    *Kenny J*

*   Perf fix: If a table has no primary key, don't repeatedly ask the database
    for it.

    *Julius de Bruijn*