Require logstash-event < 1.2.0

The logstash-event gem in version 1.2.x is not compatible with
Ruby 1.8 anymore which is currently a requirement for Rackstash.
As such, we have to use an older version.
Add coveralls
Only require the buffered_logger in buffered_logger_test to keep a clean scope
Require a compatible mime-types version in the tests

mime-types is requires by rest-client which is required by coveralls.
Only versions < 2.0.0 are compatoble with Ruby 1.8.