Update development dependencies

[mvz]

Summary

Updates development dependencies

Details

Updates dependencies to use the latest versions on modern Rubies, as far as possible. RuboCop is not updated because 0.50.0 is the last versions supporting all Ruby versions that Aruba supports.

Motivation and Context

It's good to stay up-to-date.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase withouth changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[olleolleolle]

@mvz If this passes Travis, let's get it merged!