Hack for gray conversion should no longer be necessary

[spenserblack]

Description

For the vim theme, there is currently a hack that uses Euclidean distance to get color conversions. 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!