Bug introduced in gem version v1.8.7 - not requiring initializer file in bugsnag.rake

This changeset https://github.com/bugsnag/bugsnag-ruby/compare/v1.8.6...v1.8.7#diff-1c05e1f13d0e55fa0c085142816b0d69R23 broke requiring the initializer file for config values (e.g. api_key)

`File.exist?` needs the file extension.