Hack for gray conversion should no longer be necessary
# Description

For the vim theme, there is [currently a hack that uses Euclidean distance to get color conversions](https://github.com/mjswensen/themer/blob/56f0bd2b547d495251f4a2a521338ff81d2a7c18/cli/packages/vim/lib/index.js#L5-L28).  With https://github.com/Qix-/color-convert/pull/75 merged, the next release of color-convert should make this calculation unnecessary. It should be able to be replaced with a simple `convert.hex.ansi256(hexVal)`.

Also, doing so will fix a bug I introduced in that hack (it converts colors 0-254 instead of 0-255 due to `< 255` instead of `<= 255`) 😥 

This should be a good issue for any first-time contributors!