Update dependencies
Fix no freeze shared spec allow frozen input

* Bignums under 2.1 are frozen by default
Rename shared specs to avoid duplicate inclusion

* Rspec will load any file suffixed with `_spec.rb` we load shared specs
  via devtools so duplicate inclusion can be avoided via removing the
  `_spec.rb` suffix.
Fix specs not to freeze nil
Fix specs to not access uninitialized ivars
Fix cache lookup to use lvars avoiding race
Disable mutant on rake ci