Fix a build error when using Ruby 2.7

It's the same issue as rubocop/rubocop#14526.

Author: koic

Gemfile

@@ -7,7 +7,8 @@ git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 gemspec
 
 gem 'bump', require: false
-gem 'prism'
+# FIXME: Remove when the next prism version is released.
+gem 'prism', '< 1.5.0' if RUBY_VERSION < '3.0' || RUBY_ENGINE == 'jruby'
 gem 'rake'
 gem 'rspec'
 gem 'rubocop', github: 'rubocop/rubocop'