Add workaround for a ruby exception

/usr/lib/ruby/2.0.0/rubygems/dependency.rb:98:in `latest_version?': undefined method `none?' for nil:NilClass (NoMethodError)
	from /home/anatol/sources/gem2arch/gembump.rb:99:in `dependency_suffix'
	from /home/anatol/sources/gem2arch/gembump.rb:200:in `block (2 levels) in <main>'
	from /home/anatol/sources/gem2arch/gembump.rb:198:in `each'
	from /home/anatol/sources/gem2arch/gembump.rb:198:in `block in <main>'
	from /home/anatol/sources/gem2arch/gembump.rb:188:in `each'
	from /home/anatol/sources/gem2arch/gembump.rb:188:in `<main>'