Add required_ruby_version and description to the gem's configuration

It improves the configuration based on what we discussed in clio#9

ten_years_rails.gemspec

@@ -10,9 +10,12 @@ Gem::Specification.new do |spec|
   spec.email         = ["[email protected]"]
 
   spec.summary       = %q{Companion code to Ten Years of Rails Upgrades}
+  spec.description   = %q{A set of handy tools to upgrade your Rails application and keep it up to date}
   spec.homepage      = "https://github.com/clio/ten_years_rails"
   spec.license       = "MIT"
 
+  spec.required_ruby_version = ">= 2.3.0"
+
   spec.files         = `git ls-files -z`.split("\x0").reject do |f|
     f.match(%r{^(test|spec|features)/})
   end