Make screengrab exit_on_test_failure have is_string: false

[mfurtak]

This is the most correct way of describing a boolean option right now.

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

Without describing a boolean option as is_string: false, some weirdness can happen with the value coercions.

[ohayon]

LGTM!

[gersonmdesouza]

@mfurtak Nice!