Allow ActiveSupport versions >= 3.2.8

Re: issue #11 

I changed the activesupport dependency in the Gemfile from "3.2.8" to ">=3.2.8"

Tested by running RSpec tests out-of-the-box, making the above change, updating activesupport to the latest version (4.1.8) and then running the tests again, with the same results before vs. after.