Run CI tests on Julia 1.4

[devmotion]

Should we drop, e.g., Julia 1.1?

[cpfiffer]

Yeah, I think 1.1-1.2 can be dropped everywhere in TuringLang, and we should drop 1.3 in a month or two.

[codecov]

Codecov Report

Merging #52 into master will decrease coverage by 0.67%.
The diff coverage is 90.58%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   80.90%   80.23%   -0.68%     
==========================================
  Files          12       12              
  Lines         880      865      -15     
==========================================
- Hits          712      694      -18     
- Misses        168      171       +3     

Impacted Files	Coverage Δ
src/DynamicPPL.jl	100.00% <ø> (ø)
src/contexts.jl	83.33% <ø> (ø)
src/model.jl	80.95% <80.95%> (-9.96%)	⬇️
src/prob_macro.jl	92.45% <92.15%> (-0.28%)	⬇️
src/compiler.jl	93.50% <100.00%> (-0.78%)	⬇️
src/utils.jl	41.02% <100.00%> (+8.67%)	⬆️
src/varinfo.jl	87.61% <100.00%> (-0.19%)	⬇️

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[devmotion]

The error still exists on Julia 1.4, it's just that the stable release of Julia on Travis has not been updated to 1.4 yet, so tests run on Julia 1.3 instead (see travis-ci/travis-build#1878).

[devmotion]

IMO that would be the most reasonable test setup: Julia 1.0 LTS, Julia 1 stable, and Julia master.

[mohamed82008]

Will close and re-open.

[devmotion]

The bugfix release of Libtask is not available yet (TuringLang/Libtask.jl#57), so I'm sure the test will fail again.