RuboCop Rake 0.6.0

Changes

• #33: Drop support for Ruby 2.3. (@koic)
• #34: Require RuboCop 1.0 or higher. (@koic)
• #38: Drop support for Ruby 2.4. (@tejasbubane)
• #36: Fix Rake/Desc to not generate offense for prerequisite declarations. (@tejasbubane)