Fixes issue with implicit conversion of pathname into string.

Not sure exactly what the issue is, but this seems common. I'm getting the error "no implicit conversion of Pathname into String" when running the fits rake task (ruby 2.1.2p95 & Rails 4.1.4). Easiest fix is to explicitly convert to string.

See: https://github.com/opbeat/opbeat_ruby/pull/4
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.