Fix use of run_callbacks in Rails 4

Rails 4 removed the second argument from `run_callback`.

Documentation fix.

This reverts commit 68aea16f.

This reverts commit 394ea1f6.

Allow rbx to fail in 1.9 mode

* Based on rfc2822's commit ed05dc6b85f52ad88a83bc18e1986f3b75176740.

As per discussion in issue #660, attachments using alias interpolations
for the attachment URL mistakenly get flagged as being duplicates,
although the interpolation expands to a unique location.

This commit (partially) addresses the issue by checking the attachment path
instead. However, the problem will remain for users who use
interpolations other than :class to distinguish attachment paths for
different models.

#expiring_url returns public_url when not using AWS to prevent exceptions when Fog Local is swapped for Fog AWS in development.