Use Prism 0.28+ for development (#292)

#290 was resolved in Prism 0.28.0.

Gemfile

@@ -11,10 +11,7 @@ gemspec
 gem 'bump', require: false
 gem 'bundler', '>= 1.15.0', '< 3.0'
 gem 'oedipus_lex', '>= 2.6.0', require: false
-# FIXME: This is a workaround for incompatibilities between Prism 0.24.0 and 0.25.0.
-# To upgrade to Prism 0.25+, it is necessary to investigate the following build error
-# and provide feedback to Prism.
-gem 'prism', '0.24.0'
+gem 'prism', '>= 0.28.0'
 gem 'racc'
 gem 'rake', '~> 13.0'
 gem 'rspec', '~> 3.7'