[API] Removed the `indices_boost` from the list of method arguments

Prefer `indices_boost` as a part of the DSL:

```ruby
client.search(
  index: 'articles,comments',
  body: {
    query: { query_string: {query: 'foo'} }
    indices_boost: {'articles' => 5, 'comments' => 1}
  }
)
```

Thanks @jarib!

Closes #29