Update IgnoredMethods for Lint/NumberConversion

[dvandersluis]

Follows rubocop/rubocop#8950.

Adds Rails' duration methods to the IgnoredMethods list for Lint/NumberConversion, so that calling to_i on one of them does not register an offense.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
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[dvandersluis]

/cc @koic

[koic]

Thank you for opening the PR. This change requires RuboCop 1.1.0 or higher. I'll start reviewing after the next RuboCop Rails 2.9 release that is the last version to support RuboCop 0.x series (Before 1.0). Please wait a moment.

[dvandersluis]

@koic fixed your comments, sorry for the delay!

[koic]

Thanks!