fixed typo

which caused nasty exception when JSON serialization was used

    /Users/kron/tmp/ruby/macaroons/.gems/ruby/2.1.0/gems/macaroons-0.4.0/lib/macaroons/verifier.rb:52:in `block in verify_caveats': undefined method `first_party?' for {"cid"=>"space:breakout", "vid"=>nil, "cl"=>nil}:Hash (NoMethodError)
        from /Users/kron/tmp/ruby/macaroons/.gems/ruby/2.1.0/gems/macaroons-0.4.0/lib/macaroons/verifier.rb:51:in `each'
        from /Users/kron/tmp/ruby/macaroons/.gems/ruby/2.1.0/gems/macaroons-0.4.0/lib/macaroons/verifier.rb:51:in `verify_caveats'
        from /Users/kron/tmp/ruby/macaroons/.gems/ruby/2.1.0/gems/macaroons-0.4.0/lib/macaroons/verifier.rb:36:in `verify_discharge'
        from /Users/kron/tmp/ruby/macaroons/.gems/ruby/2.1.0/gems/macaroons-0.4.0/lib/macaroons/verifier.rb:30:in `verify'