Only send notices if configured with an api_key

Currently if the gem is included, but not configured, it will intercept exceptions and attempt to send them to `api.airbrake.io` without an API key.

This changes the behaviour of send_notice to only send a notice if airbrake is configured.  This defines configured mean it has a non-blank API key.

This is desirable because we want to be able to add airbrake to our application, and have the initializer added to the app at deploy time in some deploy targets (but not others).