Add missing require of stringio

Closes #141
Bump version to 0.3.1
Fix adamantium dependency to only depend on the minor version
Bump parser dependency to ~> 2.1.0
Add conditional loop mutator

* Closes #143
* Renamed Mutator::Node::While to ConditionalLoop and use it to
  generated mutations for until also.
Improve AST coverage of mutant

* Uses a monkeypach extension to parser with a list of valid node types.
* Add nodes, found to be uncovered to generic mutator.
Remove Mutant::Support::Metho object in favor of procto
Use more modern spec file naming in spec/unit/mutator/node/*_spec.rb

* The old layout was imposed by older mutants that selected tests via
  file names.
* Reduce the useless namespace Mutator::Node::Connective
Use more modern spec layout

* This specs still have spec per method granularity
* But one file specs multiple methods
* Compatible with mutant spec selector
* Deduplicates boilerplate unit setup
Whitespace
Use devtools from git again (removes triage)
Adjust rubocop config
Remove unused setter
style
Remove old rubocop config
Bump unparser dependency to 0.1.6
Prepare version 0.3.2
Fix specs failure introduced by unparser change
Hide unparser from zombification, causes reference bug to parser
Set gem.required_ruby_version to >= 1.9.3