dev-lang/julia: Use git-2.eclass’ submodule support.

Using the eclass’ submodule support permits it to store the submodule
repositories in egit-src.  Otherwise, julia’s make has to re-clone each
of the sub-repos from scratch every emerge.

Signed-off-by: James Cloos <cloos@jhcloos.com>
Merge pull request #91 from jhcloos/julia

dev-lang/julia: Use git-2.eclass’ submodule support.