Update parser gem to eliminate Ruby 2.7.5 warning

This eliminates the warning:

warning: parser/current is loading parser/ruby27, which recognizes
warning: 2.7.4-compliant syntax, but you are running 2.7.5.

• Diff of parser: https://my.diffend.io/gems/parser/3.0.2.0/3.0.3.2
• Diff of ast: https://my.diffend.io/gems/ast/2.4.1/2.4.2