Change Relation#sort_by to work with no arguments or block

* The directions will default to the current relation header.
Add Axiom::Relation::Header#context

* Refactor context construction
Update metrics threshold
Add docs to include the header as an allowed argument to the constructor
Change Axiom::Relation#sort_by to require a block again

* Will be adding #sort method to handle the case where no arguments
  are passed and we want to sort by the header by default.
Add Axiom::Relation#sort
Move Relation#one to Limit#one
Update Relation#one to accept an optional block
Move default block to a constant
Add example for Relation#one to README
Fix rubocop to be disabled until parser dependency is fixed
Fix guard-rubocop to be disabled too
Change the default sort order to an empty Array

* By default the header is appended to any directions so using an
  empty Array has the same effect.
Upgrade gem dependencies
Move rubies back to the rvm section in travis
Add missing YARD docs
Update Relation::Base to accept a name that responds to #to_sym
Add missing encoding comment
Upgrade gem dependencies
Update comment in rubocop config