Enable autocorrection for Style/DateTime

[dvandersluis]

Style/DateTime did not have autocorrection enabled, so I added it to replace DateTime with Time. I have marked this as unsafe because we are not checking if the date being used is historic or not (unless a start parameter is passed in, such as in iso8601, in which case an offense isn't registered anyways).

I did not enable autocorrection for to_datetime because the message just states to not use it.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[marcandre]

LGTM

[dvandersluis]

@koic @marcandre would this be okay to merge now?

[koic]

The next release may be 0.91.1 (bug fix) and I'm not sure about next release version. I would like to merge when the next release is decided to be 0.92.

[dvandersluis]

@koic makes perfect sense!

[bbatsov]

I'll cut the new release later today.

[dvandersluis]

@bbatsov @koic updated for post 0.91.1.

[bbatsov]

Thanks!