Define force_ssl configuration in secrets file

this is usually configured in the production.rb file (which is under
version control), the natural place to configure it is the secrets.yml
file.

Until now we were using the capistrano shared folder, but that's a bit
inconvenient since changes we've done to the production.rb file (like
changing eager_load_paths when we upgraded to Rails 5) won't take effect
after a deployment.

config/secrets.yml.example

@@ -32,6 +32,7 @@ test:
 staging:
   secret_key_base: ""
   server_name: ""
+  force_ssl: true
   rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""
@@ -53,6 +54,7 @@ preproduction:
   #   password: "<password>"
   #   authentication: "plain"
   #   enable_starttls_auto: true
+  force_ssl: true
   rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""
@@ -79,6 +81,7 @@ production:
   #   password: "<password>"
   #   authentication: "plain"
   #   enable_starttls_auto: true
+  force_ssl: true
   rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""