[WIP] Bump compatibility for ColorTypes v0.12

[kimikage]

In ColorTypes v0.12, the definitions of one and AbstractGray have (will be) changed. Also, as of ColorTypes v0.11, some methods have been migrated from this package to ColorTypes. For these reasons, this drops the compatibility with ColorTypes v0.11 and earlier.

[codecov[bot]]

Codecov Report

Merging #171 (e895055) into master (445ce7c) will increase coverage by 6.88%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   84.93%   91.81%   +6.88%     
==========================================
  Files           2        2              
  Lines         239      220      -19     
==========================================
- Hits          203      202       -1     
+ Misses         36       18      -18     

Impacted Files	Coverage Δ
src/ColorVectorSpace.jl	97.58% <100.00%> (+7.76%)	:arrow_up:

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[kimikage]

Since master has already moved to v0.10, there is no need to give notice in this PR. Please remind this when you need it.