Merge pull request #623 from airblade/drop_ruby18_support

Specify required ruby version of 1.9 or greater

paper_trail.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |s|
   s.require_paths = ['lib']
 
   s.required_rubygems_version = '>= 1.3.6'
+  s.required_ruby_version = '>= 1.9'
 
   s.add_dependency 'activerecord', ['>= 3.0', '< 6.0']
   s.add_dependency 'activesupport', ['>= 3.0', '< 6.0']