Add new Rails/RelativeDateGrammar cop

[aeroastro]

This PR adds new Rails/RelativeDateGrammar cop.

It checks whether the word orders of relative dates
are grammatically easy to understand.
This check includes detecting undefined methods on Date(Time) objects.

# bad
tomorrow = Time.current.since(1.day)

# good
tomorrow = 1.day.since(Time.current)

Although the resulting value is the same,

• 1.day.since(Time.current) is grammatically easier to understand.
• 1.day.since(Time.current) is a natural extension of 1.day.since.

As the receiver and the argument get more complicated, this difference
becomes larger. This cop forces more understandable style, and thus,
assists easier development.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[aeroastro]

The corresponding update to Rails Style Guide is the following PR.
rubocop/rails-style-guide#345

As the guide consistently uses instances of ActiveSupport::Duration as receivers,
IMHO, this is nothing more than verbalizing an implicit convention.

[aeroastro]

Reflecting review at Rails Style Guide, I have added the following changes.

• Use Use instead of Prefer to enforce the cop rule
• Use ago and since, which are actually implemented methods in (Date)Time, in the test.
• Improve description for the Cop to include detection of undefined methods.