Add warning when checkouts contain project versions not matching declared dependencies

[hugoduncan]

When working with checkouts that are symlinks, it is very easy to checkout out a branch in your sub-project's canonical directory, or otherwise modify it, and forget that the sub-project is symlinked. This can lead to a top level project misbehaving for non-obvious reasons.

Please add a warning when a subproject in checkouts contains a project.clj file with a version different to that listed in the top level project's :dependencies.