Allow use of ActiveSupport 4.2

I upgraded a Rails application to 4.2 and my version of `DropletKit` downgraded to `1.0.1` which doesn't have support for some things I was using.

A few thoughts I'm having:

- Is ActiveSupport absolutely positively needed? If not, let's remove it!
- Can we have looser versioning constraints on ActiveSupport?

Happy Holidays.