Set target version for Performance/UnfreezeString

[tagliala]

As stated in the class documentation, unary plus operator is available from Ruby 2.3.

This fix prevent RuboCop Performance to raise an offense when configured to support Ruby 2.2

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• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Added tests.
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[koic]

Good catch. Thanks!