Fix exit to handle boolean return value from Minitest.run

Unlike Minitest::Unit.runner.run which returns an integer code indicating the total number of failed tests, Minitest.run in Minitest 5 simply returns true or false. This fixes Zeus to return 0 to indicate success and 1 to indicate failure (instead of returning the number of failed tests which is not directly accessible through the new Minitest API).

Relates to https://github.com/burke/zeus/issues/475.