Emit useful warnings in case of failed HTTP Response + travis

Hi,

I'm using this for https://metacpan.org/pod/Log::Log4perl::Appender::Raven and I whilst I was doing some real world testing, I had to monkey patch the _post_event method so I could see why my events were not going though.

This solves this issue and will hopefully be useful to users to help diagnosing what's wrong with their Sentry integration.

Additionnaly, I added a .travis.yml for continuous build checking/integration goodness.

Cheers,

Jerome.