Move to deoplete where it makes sense

[AGhost-7]

• Remove YCM from the base neovim image. To keep things DRY I'll instead just put an install script into the image. Then I can just run ycm-install --racer-completer and use whatever plugins I may need.

• Start replacing YCM stuff with deoplete completers. Candidates:

• https://github.com/sebastianmarkow/deoplete-rust Not as good as YCM at this time.

• Ensime vim already supports deoplete: https://github.com/ensime/ensime-vim/issues/282

• https://github.com/zchee/deoplete-jedi Now in use.

• https://github.com/zchee/deoplete-go

• neco-ghc already supports deoplete: https://github.com/eagletmt/neco-ghc