Remove LegacyGemResolver + LegacyEncoder

[RicardoTrindade]

Description of the change

From v3 onwards Rollbar only works with Ruby 2.0 onwards.

• Gem::GemPathSearcher no longer exists in early version of RubyGems so it's no longer needed to find a GemResolver class dynamically in RollbarRunner.
• Removes LegacyEncoder since String.encode is available from Ruby 2.0 onwards

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Maintenance
• [ ] New release

Checklists

Development

• [ ] Lint rules pass locally
• [ ] The code changed/added as part of this pull request has been covered with tests
• [ ] All tests related to the changed code pass in development

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] "Ready for review" label attached to the PR and reviewers assigned
• [ ] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer