Update shoulda-matchers.gemspec

finishing drop support to ruby 1.9.x

shoulda-matchers.gemspec

@@ -18,6 +18,6 @@ Gem::Specification.new do |s|
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.0.0'
   s.add_dependency('activesupport', '>= 4.0.0')
 end