Resolve symbolic links to load envrc in parent dirs

Hello.
I tried to fix a issue that `direnv` failed to load `.envrc` when one moved to symbolic link to directory.

That is:

    $ mkdir ~/gocode-foo/src/github.com/user/foo
    $ touch ~/gocode-foo/.envrc
    $ ln -s ~/gocode/src/github.com/user/foo ~/foo
    $ cd ~/foo
    direnv: loading ../../../../.envrc
    /bin/bash: line 133: ./.envrc: No such file or directory

This is important especially for go-lang and `layout go` users.

Thanks.