Update ruby_parser to eliminate Ruby 2.7 warning messages

Running `bin/rake gettext:regenerate` would spew:

```
NOTE: RubyParser::V27 undefined, using RubyParser::V26.
```

This commit updates the ruby_parser to v3.15.0, which brings in Ruby 2.7
support: https://github.com/seattlerb/ruby_parser/commit/b6fc793