Update powerlevel9k to 0.6.6

Changelog:

• The rbenv segment is no longer a default segment in the LPROMPT.
• Fixing issue in v0.6.5 where we changed some color codes.
• Add Tests
• test-in-docker: fix with newer ZSH versions
• [Docs] Add etc state description in dir docs
• Use SUDO_COMMAND to check for sudo
• [Bugfix] Resolve #918 Transparent background
• Fix font issue debugging script
• Add missing colors to fix color comparison
• Add fallback icon for missing linux distro icons
• Fix broken link in readme
• fallback icons for Linux distros
• etc state for dir segment in docs
• sudo detection got crazy, there. sorry, everyone.
• more default color issues.

Installed the theme and can confirm it works.