loosen mime-types requirement

`>= 1.25` spontaneously began to cause infinite loops and other
confusing failures in bundler dependency resolutions. I don't know of any
reason 1.16 wouldn't be sufficient, and that's the version required by the
mail gem, required by actionmailer.
load default i18n in spec

This may have been broken, in certain environments, by #1264
Add support for custom Paperclip mappings
Restore gem dependencies for ruby 1.8.7
Account multiple versions with interpolations

Previous commit missed the case of having multiple versions with
interpolations. Versions work by creating anonymous subclasses of the
uploader, so we need to share the mappings instance between all of those
without polluting the Paperclip module itself. Fun.