add minimal development dependencies to gemspec

we're using Gemfiles in ci/ in order to setup different contexts. this might change in the future. this commit includes development dependencies that are required to run core tests (the test setup will skip certain tests that require, e.g. rufus/tokyo)

i18n.gemspec

@@ -17,4 +17,9 @@ Gem::Specification.new do |s|
   s.require_path = 'lib'
   s.rubyforge_project = '[none]'
   s.required_rubygems_version = '>= 1.3.5'
+
+  s.add_development_dependency 'activesupport', '~> 3.0.0'
+  s.add_development_dependency 'sqlite3-ruby'
+  s.add_development_dependency 'mocha'
+  s.add_development_dependency 'test_declarative'
 end