Dropping support for Julia < 1.3, REQUIRE

Due to a change in Julia packaging infrastructure, repo2docker can no longer build repositories with Julia older than 1.3 (November, 2019), which means all versions that were supported by the deprecated Julia REQUIRE file are no longer supported, effectively removing support for the REQUIRE file itself.

If you were using REQUIRE or Julia < 1.3, you are encouraged to update Julia and use a Project.toml file, as in the julia example repo.

More information in the pull request removing it