Remove start_version and end_version

[nex3]

Once #1222 is done, I don't think it'll make much sense at all to talk about version numbers for Sass as a language, or to track them as part of our specs. Instead, I think we should focus on which features are supported by which implementations.

Dart Sass may eventually want to switch to multi-tier development, like Ruby Sass's stable/next/master distinction, but I still don't think tracking language versions is the right way to do that. Instead, I think we should extend todo and ignore_for to support implementation-level version numbers. For example:

---
:ignore_for:
- dart-sass < 1.1

would only run the test on Dart Sass 1.0.x.