Default Julia version for the REQUIRE buildpack is 0.6.4, not the latest version

[consideRatio]

If use of REQUIRE file isn't deprecated, it would make sense that not having a pinned version should lead to the latest version of julia, rather than an ancient version.

If it is deprecated though, in favor of Project.toml, it can make sense still to just retain as it is. But, then we should declare it clearly here and there.

[minrk]

Having encountered this in #1239, I think we should deprecate REQUIRE, retain it as-is and point to Project.toml.