Add Julia 1.5.0 support

[tomyun]

Julia 1.5.0 just released yesterday. Hopefully this works for Binder.

[betatim]

Our tests fail because packages like IJulia don't precompile. Not sure if this is expected or not when we upgrade Julia version.

Do you know more about what changed between 1.5 and the previous version?

[betatim]

Actually, it is the Julia legacy test (tests/julia/julialegacy_version-1.1) for v1.1. which has started failing.

@davidanthoff any ideas what recently happened in Julia world that this would stop working?

Actually, actually it seems most of the "legacy" test cases suddenly fail https://dev.azure.com/jupyter/repo2docker/_build/results?buildId=305&view=logs&j=ee64945a-faa3-5012-a8b2-5da449b6aeed&t=800caa60-10d1-50ab-ca6c-028744e4732a&l=33 takes you to a overview of the tests that pass/fail.

[davidanthoff]

Hm, good question... Maybe the Julia definition in the REQUIRE file is read as a semver version, so writing julia 1.1 in REQUIRE means "any version that is compatible with 1.1", and that of course would include 1.5. BUT, I think 1.5 might have removed support for REQUIRE files...

If that is so, we should just cap the Julia version one can get via a REQUIRE file to Julia 1.4.2. The whole REQUIRE stuff is completely legacy in any case.

[betatim]

Modified the julia buildpack locally to print out the version with julia --version. Three observations: it prints 1.0.2 for the 1.0.2 test (👍), it uses the buildpack for Project.toml based projects, and it runs to completion. So maybe this was a glitch because ... glitches happen?

Restarted the CI jobs.

[welcome]

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