handle "false" rsync ssh options

[elijh]

The current code doesn't work with some ssh options, like :paranoid => false

needed changes to rsync.rb:

- next unless value
+ next if value.nil?
...
- when :config                  then "-F '#{value}'"
+ when :config                  then value ? "-F '#{value}'" : nil

Sorry for not issuing a nice pull request, I am not using supply_drop anymore, but I am using the rsync ssh options code in https://github.com/leapcode/leap_cli.