Add `required_ruby_version` to the Gemspec.

See http://docs.rubygems.org/read/chapter/20#required_ruby_version

Some features, such as commit 4b6ada72cf, requires Ruby >= 1.8.7.
Also, there's no trace of Ruby 1.8.6 support in the .travis.yml file.

factory_girl.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |s|
     s.test_files    = all_files.select {|file| file =~ /^(spec|features|cucumber|gemfiles|Appraisals)/ }
 
     s.require_path = 'lib'
+    s.required_ruby_version = Gem::Requirement.new(">= 1.8.7")
 
     s.authors = ["Joe Ferris"]
     s.email   = %q{jferris@thoughtbot.com}