deps: fix delete-deps if deps_dir ends with dot

Remove possible last dot from deps dir path by adding
slash to value deps_dir from config.
With this fix delete-deps command deletes dependencies
properly in case of deps dir path ended with dot.
Examples of deps_dir config value that triggers error:
".", "some/path/.".
Code that triggers error: rebar_deps:'delete-deps'/2,
code line lists:prefix(DepsDir, D#dep.dir).
If deps_dir = ".", cwd = "/root", dependency name = "app"
then DepsDir = "/root/." and D#dep.dir = "/root/app" so
prefix fun returns false but should be true.
Dependency app will not be removed because of this.
If symbol "/" is added to the deps_dir path last dot will
be removed by filename:join/1 in get_deps_dir/2.