Updated ruby-neovim to the latest stable release 0.8.0.

• Allow Buffer#append to take a string with newlines
• Use non-strict line indexing in :rubydo to prevent line deletions from throwing exceptions
• Performance optimizations:
  • Cache RPC method lookups and store them in a set
  • Only flush writes before reading in the event loop
  • Delete request handlers after invoking them
  • Refresh provider globals in a single RPC request

Installed with the latest version of Neovim.
Ran ':checkhealth' with Neovim reporting that 'ruby-neovim' is installed correctly.