Update Order to allow partial headers

* Will sort by the provided headers first, then use the remaining
  headers as a "tie breaker".

Fixes #39
Fix relvar YARD docs

* Uses more common phrase for relation variables
* Fix spelling mistake in @example
* Make sure all @example code uses the same relvar variable name
Fix YARD docs variables
Add Relation::Variable#replace
Change the base relation name to be a Symbol
Change equal() spec helper to be()
Upgrade gem dependencies
Update specs to use be instead of equal matcher
Merge branch 'master' into axiom-types-refactor

Conflicts:
	config/flay.yml
	spec/unit/axiom/aggregate/count/type_spec.rb
	spec/unit/axiom/aggregate/mean/type_spec.rb
	spec/unit/axiom/aggregate/standard_deviation/type_spec.rb
	spec/unit/axiom/aggregate/variance/type_spec.rb
	spec/unit/axiom/attribute/boolean/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/boolean/class_methods/type_spec.rb
	spec/unit/axiom/attribute/class_methods/infer_type_spec.rb
	spec/unit/axiom/attribute/date_time/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/decimal/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/float/class_methods/type_spec.rb
	spec/unit/axiom/attribute/integer/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/numeric/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/object/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/object/class_methods/type_spec.rb
	spec/unit/axiom/attribute/string/class_methods/primitive_spec.rb
	spec/unit/axiom/attribute/string/class_methods/type_spec.rb
	spec/unit/axiom/attribute/type_spec.rb
	spec/unit/axiom/function/connective/type_spec.rb
	spec/unit/axiom/function/numeric/binary/type_spec.rb
	spec/unit/axiom/function/numeric/division/type_spec.rb
	spec/unit/axiom/function/numeric/exponentiation/inverse_spec.rb
	spec/unit/axiom/function/numeric/square_root/type_spec.rb
	spec/unit/axiom/function/predicate/type_spec.rb
	spec/unit/axiom/function/proposition/type_spec.rb
	spec/unit/axiom/function/string/length/type_spec.rb
	spec/unit/axiom/relation/base/name_spec.rb
	spec/unit/axiom/relation/operation/order/directions_spec.rb
	spec/unit/axiom/relation/operation/order/methods/sort_by_spec.rb
	spec/unit/axiom/relation/operation/reverse/directions_spec.rb