Cache interpolator methods and reduce memory allocations - rebased on v4.3

something in aruba 0.10.x is breaking the cucumber specs

the name string is used multiple times in interpolation so storing it reduces object creation

this commit primarily uses frozen strings to reduce object creation during interpolation.
the :basename method now uses File.basename(file, ".*") rather than a Regexp. basename may be called multiple times.

by using the attachment name (symbol) and Class as keys we reduce the number of Strings created before hitting the cache

Removes note for running paperclip in EOL'd version of Rails (2.3).

Removes `column_type` and `column_options` so that it works well in both
versions.

Backcported from master, to release a patch version with this bugfix.

[closes #1947]

Fix GeometryParser for '@>' special case

According to the ImageMagick documentation the '@' flag is a special case that
can be used in conjunction with '>' to prevent IM from scaling an image up.
The '<' flag is ignored when using '@'.

http://www.imagemagick.org/Usage/resize/#pixel

This fixes the issue I reported in #1851

Fix specs broken by aruba changes

* update aruba methods for in_current_dir and check_file_presence
* switch rspec syntax from should to expect
* configure raise_in_transactional_callbacks if AR >= 4.2
* remove paperclip itself from Appraisal
This also updates Travis to use the default bundler (to enable caching) and to build primary rubies first

[ci skip] Add Ubuntu install instructions

Update version in README

You guys missed the version update for the gemfile entry.

It should never be the case that `url` returns `nil`. If there is no url
that can be reasonable achieved, as is the case when the record the
attachment is attached to is unpersisted, then `url` should return the
default url.

For some reason the test in this PR would make all other ones blow up.
For reasons not fully understood at the time of writing, the alternate
spec I'd written doesn't. So, we'll replace that test with this one and
it'll be green.

[ci skip]

We upgraded the minimum required Ruby version to not include end-of-life
Rubies, but forgot to take into account that it's a backwards
incompatible change.

In this commit we rollback that change, allowing paperclip ~> 4.2.4 to
run on 1.9.2 again.

[fixes #1902]

A number of people were reporting that there were issues with the gem
dependency specifications changes we made recently. This will revert
them.

[ci skip]

Change it from 3.2.15 to 3.2.0. We'll let bundler handle installing the
latest version. With `~>` the last number doesn't really matter anyway.

A number of people were reporting that there were issues with the gem
dependency specifications changes we made recently. This will revert
them.

For the full details of what is included in this release, check out NEWS.

[fixes #1875]