update mime-types requirement to allow 1.x and 2.x

Hi all,

In Rails 4.2, the mail gem is updated which allows mime-types to be updated to 2.x. In my app, the only gem holding on to a mime-types 1.x requirement is stripe.

I changed the requirement to `>= 1.25 && < 3.0`. All the tests pass. I am able to run my app on Rails 4.1.4, mime-types 1.25 as well as Rails 4.2.0.alpha (on master) and mime-types 2.3.

-Nick