Fix file delete being called twice on remove

When a file is removed with something like `.remove_image = true` it's seen as you replacing the file with a new file and still tries to delete the "old file" even though the old file was just deleted. This is mitigated in `CarrierWave::SanitizedFile#delete` by an `if exists?` condition. But it's unnecessary and especially annoying when trying to use a file storage API.
RSpec 2.14 is still being used in Carrierwave so `be_true` works just fine. It's only in Rspec 3 that `be true` or `be_truthy` is preferred. You can see that `be_true` is used throughout the entire file. So I'm just sticking to the convention. I can change it if need be of course but it should probably changed everywhere at once for consistency.