Add rule for treatment of braces around last-parameter hashes

There's currently no rule governing how last-parameter hashes are treated in our code and we could have Rubocop helping us with maintaining a consistent style for future code if we agree on one standard.

I'd like you to say which of the following options you prefer:

* `braces`
  ```ruby
  # not accepted
  Foo.bar('foo', { a: 'a' }, b: 'b')

  # accepted
  Foo.bar('foo', { a: 'a' }, { b: 'b' })
  ```
* `no_braces` (this the default if the checker is turned on)
  ```ruby
  # not accepted
  Foo.bar('foo', { a: 'a' }, { b: 'b' })

  # accepted
  Foo.bar('foo', { a: 'a' }, b: 'b')
  ```
* `context_dependent`
  ```ruby
  # not accepted
  Foo.bar('foo', { a: 'a' }, b: 'b')

  # no braces because second to last parameter is NOT a hash literal
  Foo.bar('foo', b: 'b')

  # required braces because second to last parameter IS a hash literal
  Foo.bar('foo', { a: 'a' }, { b: 'b' })
  ```
My vote (as reflected in the code) is to go with the default, `no_braces`.