[Fix] U3d::Utils: replace Ruby agent with another one to bypass 403 from Unity

[niezbop]

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Pull Request Checklist

• [x] My pull request has been rebased on master
• [x] I ran bundle exec rspec to make sure that my PR didn't break any test
• [x] I ran bundle exec rubocop to make sure that my PR is inline with our code style
• [x] I have read the code of conduct

Pull Request Description

Fixes #433

Unity has started blocking the default Ruby agent, which leads u3d to face 403 Forbidden when making requests to their system. This makes it so that our ongoing requests use Mozilla's browser agent to bypass this issue.

[Foggzie]

@niezbop @lacostej Any news on this one? We'd love to have this in an official version soon.

[Foggzie]

@niezbop @lacostej Just hoping for any updates here?

[niezbop]

Sorry for the delay. Was waiting for @lacostej and forgot about u3d after that. Merging now and releasing soon.

[Foggzie]

@niezbop This works great! Thanks for the fix.