lock i18n version to ~> 0.6.0 to fix build for rails 3.1/ruby1.8.7

Detail:

The current version(released on December 19, 2014) of i18n(which rails depends on) REQUIRED RUBY VERSION: >= 1.9.3 (https://rubygems.org/gems/i18n)

However the Rails-3.1.12 that used in aepos integration test depends on i18n '~> 0.6'
(https://github.com/rails/rails/blob/3-1-stable/actionpack/actionpack.gemspec)
(https://github.com/rails/rails/blob/3-1-stable/activemodel/activemodel.gemspec)

As new version of i18n released, the version of i18n for rails 3.1.12 resolved to 0.7.0 but that version requires
ruby version >= 1.9.3 which make bundle install errors:
(https://travis-ci.org/appfolio/ae_page_objects/jobs/43120957)

Fix:
add i18n pessimistic version constraint (~> 0.6.0) under rails 3.1.12 && ruby 1.8.7