Add minimum ruby version requirement

This only specifies "1.9" rather than "1.9.3" since it appears
that JRuby in 1.9 mode reports its version as "1.9", without
the third version number segment.

mongoid.gemspec

@@ -14,6 +14,7 @@ Gem::Specification.new do |s|
   s.summary     = "Elegant Persistance in Ruby for MongoDB."
   s.description = "Mongoid is an ODM (Object Document Mapper) Framework for MongoDB, written in Ruby."
 
+  s.required_ruby_version     = ">= 1.9"
   s.required_rubygems_version = ">= 1.3.6"
   s.rubyforge_project         = "mongoid"