Set documenter version, switch to GitHub workflow for CI

[lostella]

No description provided.

[codecov-io]

Codecov Report

Merging #32 into master will decrease coverage by 1.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   91.74%   90.46%   -1.29%     
==========================================
  Files          17       17              
  Lines         424      430       +6     
==========================================
  Hits          389      389              
- Misses         35       41       +6

Impacted Files	Coverage Δ
src/arraypartition.jl	57.14% <0%> (-9.53%)	⬇️
src/syntax/variable.jl	81.81% <0%> (-8.19%)	⬇️
src/calculus/precomposeNonlinear.jl	86.95% <0%> (-3.96%)	⬇️
src/syntax/expressions/addition.jl	79.48% <0%> (-2.1%)	⬇️
src/syntax/terms/proximalOperators_bind.jl	88.13% <0%> (-1.52%)	⬇️

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[lostella]

The docs build is likely broken because matrix supersedes jobs, see e.g. one of the blue boxes here https://juliadocs.github.io/Documenter.jl/stable/man/hosting/

I replaced both Travis and Appveyor with a GitHub workflow in ProximalOperators, see https://github.com/kul-forbes/ProximalOperators.jl/blob/master/.github/workflows/ci.yml, as soon as I get the docs build working on it I can do the same here: these workflows are faster at running checks, their configuration is likely more stable than Travis’, allow you to do all in one place (GitHub).

Will update this PR with that, if there are no concerns @nantonel