Change to relax unicode-display_width version constraint to allow 3.x

[op-ct]

Describe the change

• Change to relax unicode-display_width version constraint to allow 3.x

Why are we doing this?

• unicode-display_width is now at 3.2.0
• Fixes Allow newer unicode-display_width #25

Benefits

• Fixes strings 0.2.1's version constraints from breaking gem install for gems whose deps pull in both strings and unicode-display_width 3.x

Drawbacks

• None as far as I can tell:
  • Local tests still pass with unicode-display_width 3.2.0
  • Other gems that use both now work again without ill effects.

Requirements

• Tests written & passing locally?
• Code style checked?
• Rebased with master branch?
• Documentation updated? N/A
• Changelog updated?

[op-ct]

@piotrmurach the appveyor checks passed, but it looks like the rest of the workflow is waiting on approval from "a maintainer." Can you help?