https://github.com/thoughtbot/paperclip/issues/760

This allows bundler to complete gem dependency resolution during
development. This is only relevant for paperclip developers. Please see:
https://github.com/thoughtbot/paperclip/issues/1256 and thanks to
@nkondratyev

Removes check for potential clashes in attachment paths, per discussion
in #1143.

As of 26f4d409, the default path settings no longer lead to this problem,
and the checker is more likely to generate false than true positives as
it cannot fully expand interpolations.

Add logging section to Readme

Implement djcp's tweaks

Uses secure RubyGems URL

Changed Spec::Runner to RSpec.

Gemfile.lock updated for Paperclip 3.4.1

Changed Spec::Runner to RSpec.

* since Gemfile.lock is checked in, seems like it wants to be updated following the 3.4.1 release?

Add default_url option to Quick Start example

It took me a lot of digging to figure out how to add the
`:default_url` option, but it seems a common option that many
people will need to modify. Putting it anywhere in the README
will help those who just cmd-F on the page, like me.

Allow either a hash or a Style object to set the headers.

S3#flush_writes was accessing the styles option without checking for a
proc. We can just use the instance's styles method to have it done for
us.