Pin power_assert to v2 for Ruby 2.7 (v3 requires 3.1+)

* See ruby/power_assert#61
* Same approach as ruby/irb#1135

Author: eregon

Gemfile

@@ -7,4 +7,5 @@ group :development do
   gem 'ruby-maven', :platforms => :jruby
   gem 'test-unit'
   gem 'test-unit-ruby-core', ">= 1.0.7"
+  gem 'power_assert', '~> 2.0' if RUBY_VERSION < '3.0' # https://github.com/ruby/power_assert/pull/61
 end