Update CI.yml

[rikhuijzer]

Just another attempt to bump the minimum Julia version being tested, as was also discussed here. Julia 1.0 is now broken for months and I don't see how that would change.

In effect, the tests for Julia 1.0 goes as follows:

1. Fail after a few seconds on a incompatible requirement; not executing any test.
2. Show a red cross at all places in the GitHub interface, introducing confusion for everyone.
3. Send an email to the creator of a pull request that the PR failed even though all tests pass.

Note that this test doesn't mean that people on Julia 1.0 cannot install Gadfly anymore. It just means that it's not tested in CI here.

As another argument. I think that this would help Gadfly move forward more easily. That is in line with some of Lehman's laws of software evolution (1980):

1 an E-type system [software that performs some real-world activity] must be continually adapted or it becomes progressively less satisfactory.
6 the functional content of an E-type system must be continually increased to maintain user satisfaction over its lifetime.
7 the quality of an E-type system will appear to be declining unless it is rigorously maintained and adapted to operational environment changes.

[codecov-io]

Codecov Report

Merging #1525 (94b33b0) into master (189bd36) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1525      +/-   ##
==========================================
+ Coverage   89.72%   89.74%   +0.01%     
==========================================
  Files          39       39              
  Lines        4672     4681       +9     
==========================================
+ Hits         4192     4201       +9     
  Misses        480      480              

Impacted Files	Coverage Δ
src/scale.jl	95.66% <0.00%> (+0.03%)	⬆️
src/Gadfly.jl	76.49% <0.00%> (+0.05%)	⬆️
src/guide.jl	90.90% <0.00%> (+0.12%)	⬆️

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