Update default Julia version and/or allow customization

Hi! Currently, repo2docker chooses julia v0.6.0 by default, but there are several newer versions of julia that would be better choices.

The default is set here: https://github.com/jupyter/repo2docker/blob/7552361c2b51bea73f3c2fba5e290d6b834b7f36/repo2docker/buildpacks/julia.py#L38

Better default values would be any of:

• v0.6.4 (the last 0.6 release)
• v0.7.0 (the “compatibility” release, which is mostly compatible with code written for 0.6 and code written for 1.0)
• v1.0 (the latest stable release. However lots of code and packages still aren’t 1.0 ready yet.)

I personally think setting v0.7.0 as the default makes the most sense.

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As a separate thing, it would be nice if we allowed the user to choose a julia version in the REQUIRE file, as is commonly done in julia REQUIRE files: https://github.com/JuliaLang/IJulia.jl/blob/master/REQUIRE#L1 https://github.com/JuliaPlots/StatPlots.jl/blob/master/REQUIRE#L1

This way, the user can choose between julia 0.6, 0.7, and 1.0.

If you think these should be two separate Issues, let me know and I’ll split them up! 😃

[davidanthoff]

I believe we sorted this one out as well and can close?

[NHDaly]

Alright, scrap that 😃

I think the right approach is this:

if there is a Project.toml (or JuliaProject.toml, then generally ignore any REQUIRE file. If there is no julia version info inside Project.toml, then use the latest released version of julia. If there is julia version info inside Project.toml, then use the highest version that is compatible with the constraint that is in the Project.toml.

If there is no Project.toml or JuliaProject.toml, and there is no julia version info inside REQUIRE, use the highest julia 0.6.x version.

Yes, I completely agree! This is what Kristoffer and I came up with as well (here: https://github.com/jupyter/repo2docker/pull/393#issuecomment-419558127)