Updating minimagick dependency for compatibility issues with debian testing

This causes other gems to be updated (Including Rails to 4.2). Following the Rails update some warnings (mostly on configuration) have been fixed and errors_on method no longer exists.

Pending migrations check is also giving problems, so we remove it.
Configuration and initializers for disabling animated gif versions
Including a trick for checking that a gif is an animated gif when processing images
We only do the resizing trick for gifs, so we save time for files that are not gifs
New test with an static gif to check that it creates versions
Checking that when animates gif versions are enabled everything works fine
Merge branch 'features/stop-processing-animated-gifs'