Revert "Use passive ftp mode by default"

We found out that some ftp client/server setups only allow active mode,
and we don't want to break these setups with this gem update.

This reverts commit 571ab9fc9a764b1b8c0b0e85ed8f5cb242296d6e.

Conflicts:
	lib/paperclip/storage/ftp/server.rb
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