Remove 5.0 support

[molovo]

This branch is a proposal which removes support for ZSH 5.0.2 through 5.0.8. As Zulu has grown these versions have become harder to support, and currently fail a lot of the test suite.

This PR is currently a single commit ahead of the next branch, and removes ZSH 5.0.8 from the versions tested, effectively dropping it from support. Please vote with a thumbs up or down, whether you would like to see this merged.

If voting passes, the following additional changes will be made:

• [ ] Remove version checks which are no longer required
  • [ ] https://github.com/zulu-zsh/zulu/blob/master/src/commands/upgrade.zsh#L83
  • [ ] https://github.com/zulu-zsh/zulu/blob/master/src/commands/init.zsh#L533

[molovo]

Ignore all that. Turns out I managed to fix the errors in 5.0 anyway. Ho hum.