Pin mutant to the rev with scope auto-expanding
Remove dead code
Use mutant from my fork for now
Simplify AttributeSet#get
Remove --profile option from rspec config
Kill mutations in AttributeSet
Use custom warn so we can stub it in tests and kill mutations in Attribute

  * later on Virtus.warn will be ignored in mutant config
Kill Attribute::Boolean mutation
Kill mutations in Collection#coerce via code simplification

  * We can safely assume coerced value will always respond to
    `#each_with_object` because coercer does Array(input)
Kill mutation in Attribute::Named