Validators love instances. Seriously, though, this was causing a problem
when the validator was being called and the private "send" didn't have
an instance to work on.

Update README to v4.1 for installation

This will make Travis not waiting for all builds to finish before notify
us the build result.

http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

Update link to paperclip-meta repository

Ownership of the paperclip-meta repository was transferred from @y8 to @teeparham.

Gemfile.lock.

It resulted with downloading attachment (i.e. from S3) on each
instance validation even when attachment didn't change at all.

Allows a lambda to be passed to decide what format a style should
be processed as

Travis build: retry bundle install on e.g. timeout

There are some file types that the file command doesn't understand.
Notably, files that have a format, but are ASCII-text based (like ruby,
certificates, etc), show up as text/plain. This presents a problem,
because the mime-types gem either doesn't report them at all (as for
.pem files) or reports them differently (as for .rb files).

This change allows a Paperclip.options[:content_type_mapping] hash,
which allows for a mapping of { :pem => "text/plain" } which tells the
spoof detector that this is an OK matchup and to allow it.

It is now a requirement for attachments to do one of three things:
  1. Have a content_type validation (e.g. "image/*")
  2. Have a filename validation (e.g. *.png, *.gif)
  3. Explicitly *not* have one of those validations

The intent is to make the default more secure, and you have to
explicitly reject the security of a validation in order to not have one.