Drop support for Ruby < 2.0

[cwjohnston]

Description

Drop support for Ruby < 2.0 by specifying required_ruby_version = '~> 2.0' in the gemspec.

Motivation and Context

Ruby 1.9 is EOL.

How Has This Been Tested?

Specs pass.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have updated the Changelog following the conventions laid out on Keep A Changelog

Known Caveats

Requires a major version bump in order to follow SemVer.

[portertech]

:+1: