bundle install fails over sweet_notifications dependency on Rails 3

```
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    excon-rails (~> 0.1.0) ruby depends on
      sweet_notifications (~> 0.2) ruby depends on
        railties (~> 4.0) ruby

    rails (~> 3.2.19) ruby depends on
      railties (3.2.19)
```
