Add required_ruby_version = '>= 2.3' to gemspec

We DO NOT officially support it but anything below 2.3 WILL BREAK so
it's wise to set this version requirement after all.

Refs #77

Author: solnic

dry-schema.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |spec|
   spec.executables = []
   spec.test_files = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ['lib']
+  spec.required_ruby_version = '>= 2.3'
 
   spec.add_runtime_dependency 'concurrent-ruby', '~> 1.0'
   spec.add_runtime_dependency 'dry-configurable', '~> 0.1', '>= 0.1.3'