Add 'group' argument to 'bummr update'

[emilianoLeite]

Motivation

Updating production gems is a process that should be treated more rigorously than updating 'development' or 'test' gems, and only trusting your automated tests suite can prove fatally dangerous, so this PR adds the possibility to only update gems from the supplied 'group' argument

Decisions

• Converted the argument for Outdated#outdated_gems to a options hash, so I could add a new option without breaking changes
• DRY'ed up CLI specs, isolating setup steps that were being repeated in several tests
• Purposely did not comply to some of rubocop's warnings to maintain the file's current style

Usage Example

bummr update --group development

(will only update gems from the development group)

[lpender]

Released in bummr 0.3.1

Thanks