Update ruby-regexp_parser package to 1.8.0.
### [1.8.0] - 2020-09-20 - [Janosch M«äller](mailto:janosch84@gmail.com)
### Changed
- dropped support for running on Ruby 1.9.x
### Added
- regexp flags can now be passed when parsing a `String` as regexp body
* see the [README](/README.md#usage) for details
* thanks to [Owen Stephens](https://github.com/owst)
- bare occurrences of `\g` and `\k` are now allowed and scanned as literal escapes
* matches Onigmo behavior
* thanks for the report to [Marc-Andr«± Lafortune](https://github.com/marcandre)
### Fixed
- fixed parsing comments without preceding space or trailing newline in x-mode
* thanks to [Owen Stephens](https://github.com/owst)
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