Don't use ActiveSupport 4.1.0

Currently because of https://github.com/arthurnn/apn_sender/commit/bd6b79073f667034e1d6b3adcd5a83a2632e99fb it's not possible to run with newer version of rails
```
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    apn_sender (~> 2.0.2) ruby depends on
      activesupport (< 4.1.0, >= 3.1) ruby

    activeadmin (>= 0) ruby depends on
      activesupport (4.1.5)
```
I'm not sure if activesupport 4.1.0 breaks anything (tried, all tests passes), but at least don't block > 4.1.0