Make #items and #layouts abstract
Query data source for item with identifier
Let #glob query data sources
Do not create directed graph with objects
Add directed graph (un)serializing
Add #vertex? to DirectedGraph
Let dependency tracker use item collection
Limit promises of what DirectedGraph methods return
Use only bundler as development dependency
Update Gemfile.lock
Fix test by letting ItemCollection#inspect not include object id
Always install yard and kramdown
Fix syntax error on Ruby 1.9.x
Use ddplugin 0.1
Add 2.1.0 to Travis config
Do not use safe level 4 on Ruby 2.1.0
Require rubysl on Rubinius
Send build notifications to #nanoc
Do not use rubocop from git
Update Gemfile.lock