Fix ImplicitDiv issues with dynamic ids after unclassed divs

Fixes causes/haml-lint#33

Credit to @parameme for his help with the regular expression solution

The `ImplicitDiv` linter has issues when an unclassed `div` has a preceding dynamic `id`.

The following examples will not report a lint, as I believe was the original intention of the linter.
```haml
%div{id: object.to_s}
%div[object.to_s]
```

I have confirmed that this change does not break any existing tests
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