Allow Backtrace to be a string, fix development dependencies

The documentation says that Airbrake.notify takes a string. Really, it takes an array, such as caller returns. However, someone on my team passed an empty string for non-exception related use of the notifier and it bombed. We didn't notice for some time. Seems reasonable that it do take a string, if desired.

I also had to add activerecord as a development dependency. I use rbenv and do a custom --path on the bundle install. I assume most people have activerecord in their system gems and don't notice the implicit dependency. I made it explicit.