Catch SystemExit in option_parser_spec

`Earthquake::OptionParser.new(['--help']).parse` will raise SystemExit because of [this line](https://github.com/leejarvis/slop/blob/master/lib/slop.rb#L147).
Since `rescue` catches only `StandardError` by default, [L54](https://github.com/jugyo/earthquake/blob/master/spec/earthquake_option_parser_spec.rb#L54) does not catch `SystemExit` and this spec fails.
I fixed it.