Issue #16: Strings get wrong colors

This quick fix improves the highlighting for the example in #16.

I do not think that `with_items` has anything to do with the problem here.  The reason the strings are not highlighted is that the `yamlScalar` syntax group does not allow `yamlString` inside it; this patch changes that.

A more complete solution to the problem would be to recognize `{ name: 'Jinja2', version: '2.7.1' }` as a flow hash (two keys, each with a value).  I do not think this can be done with a quick fix.