Fix environment so simplecov will not abort test suite before it is run

Requiring coveralls and thus simplecov after test/unit was causing the
test run to be aborted by simplecov's at_exit before it was run by
test/unit's at_exit.

Also:
- Added some requirements to Gemfile so that 'rake test' works by
itself (without appraisal).
- Added filter to coverage report to exclude test files.