Don't overspecify the julia version

This is to bring the julia version information into accordance with
section 7.3.2 of the nixpkgs manual:

    The version in the filename should leave out unnecessary detail.
    For instance, if we keep the latest Firefox 2.0.x and 3.5.x
    versions in Nixpkgs, they should be named firefox/2.0.nix and
    firefox/3.5.nix, respectively (which, at a given point, might
    contain versions 2.0.0.20 and 3.5.4).

Julia uses semantic versioning (http://semver.org), so the third
digit (the patch level) represents backwards-compatible bug fixes
only.