Pow Servers requires Arbre::Context#length

[gregbell]

As reported in #103 by @ambethia:
{"name":"NoMethodError","message":"undefined method length' for #Arbre::Context:0x000001029e5180","stack":"/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/bundler/gems/active_admin-0e23ccbdb170/lib/active_admin/arbre/html.rb:20:in method_missing'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:159:in block in handle'\n/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/gems/rack-1.3.0/lib/rack/lock.rb:7:in block in each'\n/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/gems/activerecord-3.1.0.rc4/lib/active_record/query_cache.rb:37:in each'\n/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/gems/activerecord-3.1.0.rc4/lib/active_record/query_cache.rb:37:in each'\n/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/gems/activerecord-3.1.0.rc4/lib/active_record/connection_adapters/abstract/connection_pool.rb:429:in each'\n/Users/ambethia/Code/grays/books/vendor/ruby/1.9.1/gems/rack-1.3.0/lib/rack/lock.rb:7:in each'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:158:in handle'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:99:in rescue in block (2 levels) in start'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:96:in block (2 levels) in start'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:86:in each'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:86:in block in start'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:66:in loop'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:66:in start'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/lib/nack/server.rb:13:in run'\n/Users/ambethia/Library/Application Support/Pow/Versions/0.3.1/node_modules/nack/bin/nack_worker:4:in

'"}`

[ambethia]

That was fast. I was just sitting down to see if I could fix this myself!