Currently `Ripper.slice` raises a FrozenError
```ruby
require 'ripper'
p Ripper.slice('foo', 'ident')
```
```
/path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:193:in `concat': can't modify frozen String (FrozenError)
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:193:in `block in compile'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:190:in `scan'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:190:in `compile'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:169:in `initialize'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:151:in `new'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:151:in `token_match'
from /path/to/g/lib/ruby/2.6.0/ripper/lexer.rb:144:in `slice'
from /tmp/tmp.kb4cnhvum2/test.rb:2:in `<main>'
```
This patch will fix the problem.
[Fix GH-1837]
From: Masataka Pocke Kuwabara <kuwabara@pocke.me>
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62743 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
67bb95936d