[119220] Preparations for CI.

There are several things here, all related to CI support:

1. Fixed test_helper requires, so it gets called before any other code (it is important).
2. For ruby > 1.8, use SimpleCov coverage library, instead of RCov. RCov is not maintained and does not work for newer Ruby versions. SimpleCov does not work for older...
3. Coverage is only calculated if either `CI_SERVER` or `coverage` environment flag is set.
4. Use ci_reporter to generate XML reports that CI server can understand. It is only run if `CI_SERVER` environment flag is set.

Also note that there are some workarounds required for ruby < 1.9. As those versions are [no longed supported](https://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/), I think we might consider dropping support, too.