Lock Suspenders to a Rails version

This is intended to help solve the error people are seeing by having Rails
4.1.2rc1 on their machine:

```
thoughtbot/suspenders/lib/suspenders/actions.rb:7:in
replace_in_file': "config.assets.version = '1.0'" not found in
config/environments/production.rb (RuntimeError) from
thoughtbot/suspenders/lib/suspenders/app_builder.rb:94:in
setup_asset_host' from
gems/railties-4.1.2.rc1/lib/rails/generators/app_base.rb:133:in
build'
```

I don't think it will totally solve the issue but it is at least more
declarative. To totally solve it, I think we need to fail fast when the script
runs by checking for `rc` in the `gem list railties` output or something.