Fix false positive/negative in Security/Eval cop

[pocke]

Changes

• Add an offense for binding.eval(something)
• Not add an offense for dstr if it doesn't have a string interpolation

 # bad
binding.eval(something)
eval <<-END
  #{foo}
END

 # good
eval <<-END
  foo
  bar
END

---

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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).