Loosened the version required for select2-rails.

I was unable to install spree edge because of a dependency conflict between versions of sprockets. This fix allows for a later version of select2-rails to be used that is not dependent on sass-rails, which is what is dependent on sprockets.

```shell
Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    spree (>= 0) ruby depends on
      spree_backend (= 2.3.0.beta) ruby depends on
        select2-rails (~> 3.4.7) ruby depends on
          sass-rails (>= 0) ruby depends on
            sprockets (~> 2.0.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.0) ruby depends on
          sprockets-rails (~> 2.0) ruby depends on
            sprockets (2.12.1)

Bundler could not find compatible versions for gem "sprockets-rails":
  In Gemfile:
    spree (>= 0) ruby depends on
      spree_backend (= 2.3.0.beta) ruby depends on
        select2-rails (~> 3.4.7) ruby depends on
          sass-rails (>= 0) ruby depends on
            sprockets-rails (~> 2.0.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.0) ruby depends on
          sprockets-rails (2.1.3)

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    spree (>= 0) ruby depends on
      spree_backend (= 2.3.0.beta) ruby depends on
        select2-rails (~> 3.4.7) ruby depends on
          sass-rails (>= 0) ruby depends on
            railties (~> 3.1.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.0) ruby depends on
          railties (4.1.0)
```
'~> 3' is too loose.  I could see '~> 3.4' being okay, but wouldn't want this allowing a major version bump without testing it out first.