Use broadcast rules on 0.6 (fixes dot-operators depwarn)

[timholy]

This is a breaking change, because on Julia 0.6 the array eltype no longer "wins": the output is the same as what you'd get from adding the scalars. The package's behavior is now consistent with whatever the Julia version does for other "number" types.

[timholy]

Test failure on nightly is because it's behind julia master.

[codecov-io]

Codecov Report

Merging #71 into master will decrease coverage by -0.24%.
The diff coverage is 57.89%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   64.68%   64.44%   -0.24%     
==========================================
  Files           1        2       +1     
  Lines         269      270       +1     
==========================================
  Hits          174      174              
- Misses         95       96       +1

Impacted Files	Coverage Δ
src/ColorVectorSpace.jl	66.83% <57.14%> (+2.15%)	✅
src/dots.jl	58.1% <58.1%> (ø)

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