Capistrano v3 tasks configurable rails_env

Currently the cap v3 tasks use the Capistrano stage as the `RAILS_ENV` when executing rake tasks.

My Capistrano v3 `RAILS_ENV` and `:stage` variables do not always match.

This PR allows users to use a new configuration variable `:thinking_spinx_rails_env` which defaults to the value of `:stage`

For example:

```ruby
set :stage, 'foo'
set :thinking_spinx_rails_env, 'production'
```