Fix mini_magick convert

MiniMagick.convert by default only converts the first page of tif files to pdf when using below code.

```ruby
class TestUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  version :pdf do
    process convert: 'pdf'
  end
end
```

After reading throw MiniMagick, I realised that mini_magick makes a poor assumption that by default it will only convert first page. So I had to use below code to convert all pages.

```ruby
module CarrierWave
  module MiniMagick
    def convert_all_pages_to_pdf
      manipulate! do |img|
        img.format('pdf', nil)
        img = yield(img) if block_given?
        img
      end
    end
  end
end
```

This PR fixes convert to format all pages by default and adds an optional page number parameter to convert specific pages

```ruby
process convert: 'pdf' #Convert all pages
process convert: ['pdf', 0] #Convert first page
```