Bump compat for CategoricalArrays to 0.8

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• bumps compat for CategoricalArrays to 0.8
• and DataFrames to 0.21

[codecov-commenter]

Codecov Report

Merging #1447 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   89.38%   89.40%   +0.02%     
==========================================
  Files          39       39              
  Lines        4379     4379              
==========================================
+ Hits         3914     3915       +1     
+ Misses        465      464       -1     

Impacted Files	Coverage Δ
src/dataframes.jl	97.56% <100.00%> (ø)
src/misc.jl	65.06% <100.00%> (ø)
src/mapping.jl	85.84% <0.00%> (+0.94%)	⬆️

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[Mattriks]

Once this is merged, we should do a new release in about a week or so.

[bjarthur]

or... wouldn't it have been better to tag a release before this was merged so that people who do not want to upgrade CategoricalArrays and DataFrames can get the latest Gadfly features and bug fixes. is it possible to tag a release of a commit that is not master?

[Mattriks]

The compats have to be bumped sometime! My thoughts were that the imminent minor release (1.3) should include this compat bump to CategoricalArrays 0.8. Also the next minor release (1.4) will probably include DataFrames 1.0.

[bjarthur]

could have had it both ways by tagging 1.3 before this merge, and then 1.3.1 immediately after. dependency compatibility is a huge pain sometimes so it's important to give people both options.