add local my_crm_credentials.rb configuration file to .gitignore

REVIEW

The configuration file `config/my_crm_credentials.rb` is only a local configuration file that should not be checked in. To prevent mistakes the file is added to `.gitignore`.

A more standard way of handling local configuration files, would be to configure the CRM via a `config/crm_credentials.yml` file, that is ignored like any other configuration file. The `config/initializers/crm_credentials.rb` would then have to be adjusted to either read the `ENV['CRM_API_KEY']` and `ENV['CRM_LOGIN']` or the `crm_credentials.yml` file. This means, the server also needs to provide a `CRM_LOGIN` environment variable.

CRUISE

https://dcc.infopark.de/project/show_build/895