Does not play well with Ruby 2 YAML serialization

The encoder is not correctly initialized when insensitive hash objects are deserialized using psych which is shipped with Ruby 2. Here is an example on Ruby 2.3.1p112:

```
irb(main):011:0> Psych.load(Psych.dump({a: 1}))
=> {:a=>1}
irb(main):012:0> Psych.load(Psych.dump({a: 1}.insensitive))
NoMethodError: undefined method `call' for nil:NilClass
Did you mean? caller
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/insensitive_hash-0.3.3/lib/insensitive_hash/insensitive_hash.rb:218:in `encode'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/insensitive_hash-0.3.3/lib/insensitive_hash/insensitive_hash.rb:209:in `lookup_key'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/insensitive_hash-0.3.3/lib/insensitive_hash/insensitive_hash.rb:127:in `delete'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/insensitive_hash-0.3.3/lib/insensitive_hash/insensitive_hash.rb:87:in `[]='
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:362:in `block in revive_hash'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:336:in `each'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:336:in `each_slice'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:336:in `revive_hash'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:271:in `block in visit_Psych_Nodes_Mapping'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:268:in `each'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:268:in `each_slice'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:268:in `visit_Psych_Nodes_Mapping'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/delayed_job-4.0.0/lib/delayed/psych_ext.rb:133:in `visit_Psych_Nodes_Mapping_with_class'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/visitor.rb:16:in `visit'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/visitor.rb:6:in `accept'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:32:in `accept'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:311:in `visit_Psych_Nodes_Document'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/visitor.rb:16:in `visit'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/visitor.rb:6:in `accept'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/visitors/to_ruby.rb:32:in `accept'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych/nodes/node.rb:38:in `to_ruby'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/2.3.0/psych.rb:253:in `load'
from (irb):12
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-3.2.22.2/lib/rails/commands/console.rb:47:in `start'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-3.2.22.2/lib/rails/commands/console.rb:8:in `start'
from /Users/ryan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-3.2.22.2/lib/rails/commands.rb:41:in `'
from script/rails:6:in `require'
from script/rails:6:in `'
irb(main):013:0>
```

Notice that this is not an issue on ruby 1.9.3p551:

```
irb(main):001:0> Psych.load(Psych.dump({a: 1}))
=> {:a=>1}
irb(main):002:0> Psych.load(Psych.dump({a: 1}.insensitive))
=> {:a=>1}
```

I am using version 0.3.3 of this gem.