Update parser gem for Ruby 2.7.6 support
We'll be updating the CI images to Ruby 2.7.6 and perhaps another version soon. This removes a warning.
Diff: https://github.com/whitequark/parser/compare/v3.0.3.2..v3.1.2.0
CHANGELOG: https://github.com/whitequark/parser/blob/master/CHANGELOG.md
Edited by Stan Hu