Add Rails/I18nLocaleTexts match for redirect_back

[bensheldon]

Rails/I18nLocaleTexts was introduced in #643 and matches on redirect_to; this PR extends the cop to also match on the similarly behaved redirect_back method.

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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.

[koic]

The following test fails.

% bundle exec rake spec
(snip)

rspec './spec/project_spec.rb[1:2:4:1:7]' # RuboCop Rails Project Changelog future entries For /Users/koic/src/github.com/rubocop/rubocop-rails/spec/../changelog/add_i18nlocaletext_match_for_redirect_back.md starts with `new_`, `fix_`, or `change_`

Can you rename this filename to changelog/change_add_i18nlocaletext_match_for_redirect_back.md?

[bensheldon]

Oops! Just renamed and force-pushed.

[koic]

Nice 👍