Set documenter version, switch to GitHub workflow for CI

[lostella]

No description provided.

[codecov-io]

Codecov Report

Merging #15 into master will decrease coverage by 0.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage   87.36%   86.70%   -0.67%     
==========================================
  Files          48       48              
  Lines        1631     1662      +31     
==========================================
+ Hits         1425     1441      +16     
- Misses        206      221      +15     

Impacted Files	Coverage Δ
src/linearoperators/Zeros.jl	60.00% <0.00%> (-9.24%)	⬇️
src/calculus/Scale.jl	76.19% <0.00%> (-3.81%)	⬇️
src/calculus/Reshape.jl	88.88% <0.00%> (-3.42%)	⬇️
src/calculus/Jacobian.jl	85.29% <0.00%> (-2.59%)	⬇️
src/calculus/HCAT.jl	71.55% <0.00%> (-0.83%)	⬇️
src/calculus/VCAT.jl	58.71% <0.00%> (-0.34%)	⬇️
src/utilities/block.jl	0.00% <0.00%> (ø)
src/linearoperators/FiniteDiff.jl	100.00% <0.00%> (ø)
src/linearoperators/ZeroPad.jl	97.67% <0.00%> (+0.11%)	⬆️
src/linearoperators/Variation.jl	97.36% <0.00%> (+0.14%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fe2c746...9c2a03e. Read the comment docs.

[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

[nantonel]

All looks good to me. Using Github CI sounds like a good idea!