Limit ActiveSupport dependency to v3

Rails 4 (and thus ActiveSupport 4) was recently released. New gem installations will try to use ActiveSupport 4, which requires Ruby 1.9.3+. This makes bluepill dependent on Ruby 1.9.3+ as well (even though it works fine on 1.8.7 and 1.9.2, versions that ActiveSupport 3.* are compatible with).

Locking bluepill's dependency to 3.* versions of ActiveSupport resolves this issue.