Add relation variables

* This branch will add a relation variable that will wrap a
  relation and propagate writes then update its reference to use the
  new relation.
Add Relation::Variable
Add Relation::Variable::Materialized
Merge pull request #38 from dkubb/relation-variables

Relation variables
Remove pelusa gem dependency
Merge branch 'master' into axiom-types-refactor

Conflicts:
	config/flay.yml