Support heredoc squiggly syntax

### Input

Ruby 2.3 supports the following syntax:

```ruby
documentation = <<~HEREDOC.strip_heredoc
  hello world

  version: #{current_version}

  ----------------------------------------------------

  Usage:
    - etc
    - etc
    - etc
HEREDOC
```

### Current output

```ruby
documentation = 'hello world
'.strip_heredoc
```

### Expected output

Same as input