Remove rollbar gem dependency

We already support Errbit and Airbrake as error monitoring services.
Since some people might not want to setup Errbit and might prefer
Rollbar over Airbrake, we're referencing it in the custom gemfile.

config/secrets.yml.example

@@ -44,7 +44,6 @@ staging:
   errbit_project_key: ""
   errbit_project_id: 1
   errbit_self_hosted_ssl: false
-  rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""
   managers_url: ""
@@ -71,7 +70,6 @@ preproduction:
   errbit_project_key: ""
   errbit_project_id: 1
   errbit_self_hosted_ssl: false
-  rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""
   managers_url: ""
@@ -103,7 +101,6 @@ production:
   errbit_project_key: ""
   errbit_project_id: 1
   errbit_self_hosted_ssl: false
-  rollbar_server_token: ""
   http_basic_username: ""
   http_basic_password: ""
   managers_url: ""